Optimization of benzoxazole-based inhibitors of Cryptosporidium parvum inosine 5′-monophosphate dehydrogenase

Abstract

Cryptosporidium parvum is an enteric protozoan parasite that has emerged as a major cause of diarrhea, malnutrition and gastroenteritis as well as posing a potential bioterrorism threat. C. parvum synthesizes guanine nucleotides from host adenosine in a streamlined pathway that relies on inosine 5′-monophosphate dehydrogenase (IMPDH). We have previously identified several parasite-selective C. parvum IMPDH (CpIMPDH) inhibitors by high-throughput screening. In this paper, we report the structure-activity relationship (SAR) for a series of benzoxazole derivatives with many compounds demonstrating CpIMPDH IC₅₀ values in the nanomolar range and > 500-fold selectivity over human IMPDH (hIMPDH). Unlike previously reported CpIMPDH inhibitors, these compounds are competitive inhibitors versus NAD⁺. The SAR study reveals that pyridine and other small heteroaromatic substituents are required at the 2-position of the benzoxazole for potent inhibitory activity. In addition, several other SAR conclusions are highlighted with regard to the benzoxazole and the amide portion of the inhibitor, including preferred stereochemistry. An x-ray crystal structure of a representative E•IMP•inhibitor complex is also presented. Overall, the secondary amine derivative 15a (Q67) demonstrated excellent CpIMPDH inhibitory activity (IC₅₀ = 0.5 ± 0.1 nM) and moderate stability (t_1/2 = 44 min) in mouse liver microsomes. Compound 73, the racemic version of 15a, also displayed superb antiparasitic activity in a Toxoplasma gondii strain that relies on CpIMPDH (EC₅₀ = 20 ± 20 nM), and selectivity versus a wild-type T. gondii strain (200-fold). No toxicity was observed (LD₅₀ > 50 μM) against a panel of four mammalian cells lines.

Introduction

Cryptosporidiosis is an intestinal diarrheal disease most commonly caused by Cryptosporidium parvum and hominis. These protozoan parasites are widely distributed in both the developing and developed worlds¹ and a major cause of severe diarrhea and malnutrition in children². Infection is self-limiting in immune-competent adults, but chronic in immunocompromised individuals and children. Infections are transmitted by the fecal to oral route through drinking and recreational waters³. The infectious oocysts are resistant to standard water treatment methods like bleach and filtration. C. parvum oocysts are readily obtained from infected calves, and could potentially be used as a bio-weapon ⁴. Hence, C. parvum is categorized as a class B bio-warfare agent. Furthermore, currently available drugs are not effective for treating cryptosporidiosis and vaccine therapy is lacking⁵, so new drugs are needed.

Cryptosporidium are intracellular parasites. Genomic analysis revealed that these parasites cannot synthesize purine nucleotides de novo, but instead salvage adenosine from the host⁶. Adenosine is converted into guanine nucleotides in a streamlined pathway that relies on inosine 5′-monophosphate dehydrogenase (IMPDH) catalyzing the conversion of inosine 5′-monophosphate (IMP) to xanthosine 5′-monophosphate (XMP) ⁷. The parasite enzyme (CpIMPDH) is structurally distinct from mammalian IMPDHs. Cryptosporidium appears to have obtained its IMPDH gene from bacteria via lateral gene transfer^{6b, 6c, 8}. Therefore, CpIMPDH has emerged as an attractive molecular target for the development of effective therapeutics for the treatment of diseases associated with this recalcitrant organism.

As previously reported, a high throughput screening campaign identified structurally diverse selective CpIMPDH inhibitors⁹. Several of these compound series have been optimized to produce low nanomolar CpIMPDH inhibitors such as A110, C64, C97, P96 and D48 (Figure 1)¹⁰. Herein, we report the structure-activity relationship for the benzoxazole-based inhibitor 1, a co-crystal structure of a representative derivative of this compound series with CpIMPDH and anti-parasitic activity for a subset of compounds in a Toxoplasma gondii surrogate model.

Figure 1.

Optimized CpIMPDH inhibitors A110, C64, C97, P96 and D48 and CpIMPDH inhibitor 1 identified by HTS.

Results and Discussion

Chemistry

Various derivatives of 1 were synthesized using the methods depicted in Scheme 1. 5-Nitro-2-arylbenzoxazoles 3 were prepared from 2-amino-4-nitrophenols 2 and aromatic aldehydes in the presence of activated carbon (DarcoKB)¹¹. Reductive hydrogenation of 3 in the presence of 10% Pd-C provided 5-amine-2-arylbenzoxazoles 4. Substituted phenols and anilines were converted to the corresponding ethers and amines 6 upon treatment with ethyl 2-bromopropionates in the presence of K₂CO₃. Enantiomerically pure phenyl ethers were synthesized by using Mitsunobu reaction conditions with (+)-methyl D-lactate or (-)-methyl L-lactate^10a. Ester hydrolysis was carried out in THF with 3M sodium hydroxide for racemic esters and with 3N HCl in THF for enantiomerically pure esters to yield acids 7. Subsequently, 7 was coupled with 5-amine-2-arylbenzoxazoles 4 with the aid of EDCI•HCl in anhydrous DMF yielding amides 8. N-alkylation of the amide was carried using sodium hydride and iodomethane to give 9 in moderate yield.

Scheme 1.

General procedure for the synthesis of benzoxazole derivatives.

Reagents and conditions: (a) R₁-CHO, O₂, DarcoKB, xylene, 120 °C, 6 h, 70–80% (b) H₂ (1 atm), 10% Pd-C, EtOAc, 6 h, 80–86% (c) R₃CHBrCOOR₄, K₂CO₃, DMF, rt, 5 h, 85–92% when Y = O (or 70 °C, 3 h, 36% when Y = NH) (d) (R)- or (S)- R₃CH(OH)COOR₄, 0 °C, PPh₃, 10 min, DEAD, rt, 12 h, 75–84% (e) 3 M NaOH, THF:H₂O (2:1), 80 °C, 3 h, 85–95% (f) 3N HCl, THF, 70 °C, 6 h, 60–70% (g) EDCI•HCl, DMF, 0 °C - rt, 12 h, 63–79% (h) MeI, NaH, THF, 0 °C - rt, 1.5 h, 48%.

Enantiomerically pure benzoxazole amines were prepared using the method outlined in Scheme 2. L-alanine (10) was allowed to react with 2,3-di-chloroiodobenzene in the presence of CuI and Cs₂CO₃ to give 11 ¹². The acid was coupled with a benzoxazole amine derivative with the aid of EDCI•HCl to give 15a. Copper (II) acetate mediated coupling of L-alanine methyl ester with 1-naphthyl boronic acid (12) yielded 13¹³. Hydrolysis under mild basic conditions generated acid 14, which was subsequently coupled with a 5-amine-2-(4-pyridyl)benoxazole to yield 15b.

Scheme 2.

General procedure for the synthesis of enantiomerically pure secondary amines of 5-benzoxazoles.

Reagents and conditions: (a) 2,3-di-ClPhI, Cs₂CO₃, CuI, DMF, 90 °C, 48 h, 57% (b) L-alanine methyl ester hydrochloride, Cu(OAc)₂, DCM, TEA, oxygen, 4Å molecular sieves, rt, 48 h, 34% (c) 1 M NaOH, MeOH, rt, 12 h, 67% (d) 4, EDCI•HCl, DMF, 0 °C - rt, 12 h, 60–70%.

An analogue of 1 lacking the ether oxygen was prepared following the synthetic procedure outlined in Scheme 3. 2,3-Dichlorophenylacetic acid (16) was treated with thionyl chloride to give the corresponding acid chloride 17. (R)-4-Benzyl-2-oxazolidinone (18) was deprotonated with n-butyllithium at −78 °C, and then the generated anion was quenched with 17 to give 19¹⁴. Diastereoselective methylation of 19 was carried out by treating this substrate with 1M solution of sodium bis(trimethylsilyl)amide at −78 °C, followed by the addition of iodomethane to give 20 with excellent diastereoselectivity¹⁴. Hydrolysis of 20 with lithium peroxide at lower temperature gave acid 21, which was subsequently converted to amide 22.

Scheme 3.

Synthesis of a 2-phenylpropionamide benzoxazole derivative.

Reagents and conditions: (a) SOCl₂, 90 °C, 2 h (b) 18, n-BuLi (2.5 M), THF, −78 °C to 0 °C, 1 h, 80% (c) (i) NaN(TMS)₂ in THF, −78 °C, 1 h (ii) MeI, THF, −78 °C, 2 h (iii) rt, 5 h, 77% (d) Li₂O₂, THF, H₂O, 0 °C, 1 h, 87% (e) 4 (R₁ = 4-Py, X = H), EDCI•HCl, DMF, 0 °C - rt, 12 h, 70%.

Replacement of the amide functional group of 1 with a bioisosteric 1,2,3-triazole was also investigated. This strategy had previously been successful with one structural class of CpIMPDH inhibitors^10a, but not with another.^10d The 1,2,3-triazole derivative of 1 was prepared according to the method depicted in Scheme 5. Propargyl ether 28 was synthesized using a Mitsunobu procedure. Then 2-(thiazol-5-yl)benzo[d]oxazol-5-amine (4, R₁ = 5-thiazole, X = H) was converted to the corresponding diazonium chloride followed by quenching with sodium azide to yield 29. Finally, a CuI mediated reaction of alkyne 28 with azide 29 gave 30 ¹⁵.

Scheme 5.

Synthesis of a 1,2,3-triazole derivative.

Reagents and conditions: (a) CH₃CH(OH)C CH, PPh₃, 0 °C, 10 min then DEAD, THF, 70 °C, 20 h, 69% (b) NaNO₂, HCl, H₂O, NaN₃, -5 °C – 0 °C, 1.5 h, 89% (c) CH₃CN, DIPEA, CuI, rt, 50 min, 84%.

An analogue in which the benzoxazole ring was replaced with a 1,3-diamide was generated using the method outlined in Scheme 6. 3-Nitroaniline 31 was coupled with isonicotinic acid to yield amide 32. Tin mediated reduction of 32 gave 33, which was coupled with (S)-2-(3,4-di-chlorophenoxy) propionic acid to generate 34.

Scheme 6.

Synthesis of an 1,3-diamide derivative.

Reagents and conditions: (a) 4-PyCO₂H, EDCI•HCl, DCM, 0 °C - rt, 12 h, 68% (b) SnCl₂•H₂O, EtOH, 70 °C, 4 h, 62% (c) (S)- 2,3-di-Cl-PhOCH(CH₃)COOH, EDCI•HCl, DCM, 0 °C - rt, 12 h, 63%.

Regioisomers of 1 were prepared using the procedures outlined in Scheme 8. 2-Amino-5-nitrophenol (37) and aromatic aldehydes were condensed and oxidized in the presence of activated carbon (DarcoKB) to give 38¹¹. Reductive hydrogenation of 38 in presence of 10% Pd-C provided 6-amino-2-aryl benzoxazoles 39. Subsequently, amines 39 were coupled with (S)-2-(2,3-di-chlorophenoxy) propionic acid with the aid of EDCI•HCl to yield 40.

Scheme 8.

General procedure for the synthesis of 6-substituted benzoxazole derivatives.

Reagents and conditions: (a) RCHO, O₂, DarcoKB, xylene, 120 °C, 6 h, 75% (b) H₂ (1 atm), 10% Pd-C, EtOAc, rt, 6 h, 85% (c) (S)-2,3-di-Cl-PhOCH(CH₃)COOH, EDCI•HCl, DMF, 0 °C - rt, 12 h, 76%.

Evaluation of CpIMPDH inhibition

CpIMPDH and hIMPDH2 were expressed and purified as previously described (note that C. hominis IMPDH is identical to CpIMPDH)¹⁶. Enzymatic activity was assayed by monitoring the production of NADH₉. IC₅₀ values reported herein were determined from the average of three independent experiments, unless otherwise noted. IC₅₀ values were also measured in the presence of 0.05% fatty acid free bovine serum albumin (BSA) in order to assess nonspecific protein binding¹⁷. Our previous experience indicates that IC₅₀ values in the presence of BSA are a better predictor of antiparasitic activity¹⁸. None of the compounds significantly inhibited hIMPDH2 (IC₅₀ >5μM).

Initially the aryl ether substituent was evaluated (Table 1). Removal of the 4-chloro group (41) resulted in a two-fold increase in inhibitory activity, whereas removal of the 2-chloro group (42) gave a two-fold loss in activity. Removal of both chlorine atoms resulted in activity comparable to the parent compound 1. The electron donating substitution 4-OMe (44) demonstrated comparable activity to 41. Translocation of the chlorine atom in 41 from the 2-position to the 3-position (45) was well tolerated. Combining these changes by utilizing a 2,3-di-chloro substituted phenyl ether (46) gave a five-fold increase in activity compared to 41 and 45. However, the 2,6-di-chloro phenyl ether derivative 47 was devoid of activity. Transforming the 2,3-dichlorophenyl into a 1-naphthyl resulted in a potent inhibitor (48) with an IC₅₀ value of 9 nM, which only increased slightly in the presence of BSA. These results suggest that the sterics of this group are an important determinant for binding. However, addition of a chlorine atom to the naphthyl group in the 4-position (49) was detrimental.

Table 1.

SAR of phenyl ring of 1 for CpIMPDH inhibition.

ID	R1	IC50 (nM)
		(−) BSA	(+) BSA
1	2,4-di-ClPh	44 ± 8	120 ± 10
41	2-ClPh	19 ± 2	50 ± 10
42	4-ClPh	105 ± 17	118 ± 3
43	Ph	40 ± 5	50 ± 20
44	4-OMePh	28 ± 2	34 ± 5
45	3-ClPh	20 ± 7	32 ± 8
46	2,3-di-ClPh	3 ± 1	11 ± 1
47	2,6-di-ClPh	>5000*	n.d
48	1-naphthyl	9 ± 3	14 ± 6
49	1-(4-Cl-naphthyl)	27 ± 1	53 ± 9

^*One determination, n.d. = not determined.

Next, the importance of amide methylene group was examined (Table 2). Removal of the α-methyl (50) or addition of another α-methyl (51) at this position resulted in complete loss of activity. Replacing the methyl with a larger isopropyl (52) resulted in approximately a three-fold loss of activity compared to 48. The enantiomers of 48 were also investigated and found to demonstrate a stereochemical preference. The (S)-isomer (54, IC₅₀ = 6.1 nM, IC₅₀ = 8 nM in the presence of BSA) was significantly more potent than the (R)-isomer (53, IC₅₀ = 400 nM). Again the (S)-isomers of several other derivatives containing electron withdrawing groups at the 2,3-positions of the phenyl ether (55, 56 and 57) demonstrated excellent CpIMPDH inhibitory activities (IC₅₀ < 10 nM). However, the (S)-isomer of the 2,3-di-methoxy derivative 58 was less active (IC₅₀ = 50 nM).

Table 2.

SAR of amide α-position of 1 for CpIMPDH inhibition.

ID	X	R1	IC50 (nM)
			(−) BSA	(+) BSA
50	CH2	2,4-di-ClPh	>5000*	n.d
51	CMe2	4-ClPh	>5000*	n.d
52	CH-i-Pr	1-naphthyl	24 ± 2	57 ± 5
53	(R)-CHMe	1-naphthyl	400 ± 70	380 ± 60
54	(S)-CHMe	1-naphthyl	6.1 ± 0.5	8 ± 3
55	(S)-CHMe	2,3-di-ClPh	1.2 ± 0.2	3.5 ± 0.7
56	(S)-CHMe	2-Cl,3-CF3Ph	9 ± 1	52 ± 5
57	(S)-CHMe	2-Cl,3-NO2Ph	2.3 ± 0.9	6 ± 4
58	(S)-CHMe	2,3-di-OMePh	50 ± 10	60 ± 20

^*One determination, n.d. = not determined.

The importance of the pyridine substituent was subsequently examined (Table 3). Replacing the pyridine with phenyl (59) resulted in complete loss of activity (IC₅₀ > 5 YM). Translocation of the nitrogen to the 2-or 3-positions (60 and 61) resulted in a 3- to 4-fold loss in potency. However, evaluation of derivatives 62 and 63 indicated that a 5-thiazole was an excellent replacement of the 4-pyridyl, while the 2-thiazole (65) and 2-pyrrole (66) derivatives resulted in loss of activity compared to 55 and 54, respectively. Finally, the 2-pyrimidine derivative 67 proved to be inactive.

Table 3.

SAR of the pyridine ring of 1 for CpIMPDH inhibition.

ID	X	R1	R2	IC50 (nM)
				(−) BSA	(+) BSA
59	Me	2,4-di-ClPh	Ph	>5000*	n.d
60	Me	2,4-di-ClPh	3-Py	150 ± 20	600 ± 200
61	Me	2,4-di-ClPh	2-Py	210 ± 20	800 ± 200
62	(S)-Me	2,3-di-ClPh		1.4 ± 0.3	2.3 ± 0.4
63	(S)-Me	1-naphthyl		2.7 ± 0.7	4.0 ± 0.9
64	Me	1-(4-Cl-naphthyl)		28 ± 6	70 ± 10
65	(S)-Me	2,3-di-ClPh		17 ± 6	30 ± 10
66	(S)-Me	1-naphthyl		26 ± 5	50 ± 5
67	Me	1-naphthyl		>5000*	n.d

^*One determination, n.d. = not determined.

Following initial assessments of the phenyl ether, the amide α-methylene and the pyridine moieties in inhibitor 1, a variety of other changes were explored for the amide and central heterocycle (Table 4). For example, bioisosteric replacement of the amide with a 1,2,3-triazole (30), N-methylation of the amide nitrogen (9) or inversion of the amide (36) all resulted in loss of inhibitory activity. However, regioisomers produced by transposing the amide from the 5-position of the benzoxazole to the 6-position (40a and 40b), surprisingly, resulted in compounds with excellent CpIMPDH inhibitory activity (IC₅₀ < 10 nM), although the relative effect of BSA appeared to be slightly greater compared to 55 and 62, respectively. Addition of a chlorine atom to the 7-position of the benzoxazole of 55 resulted in 68 and significantly eroded CpIMPDH inhibitory activity. Finally, replacing the benzoxazole with a benzimidazole (26) or opening of the oxazole ring (34) lead to loss of inhibitory activity.

Table 4.

Other miscellaneous changes to explore the SAR of 1 for CpIMPDH inhibition.

ID	Structure	IC50 (nM)
		(−) BSA	(+) BSA
30		>5000*	n.d.
9		>5000*	n.d.
36		>5000*	n.d.
40a		0.6 ± 0.5	4 ± 2
40b		7 ± 1	60 ± 20
68		240 ± 90	600 ± 300
26		>5000*	n.d.
34		>5000*	n.d.

^*One determination, n.d. = not determined.

The next perturbation examined was to replace the ether C-O bond with a C-C bond (Table 5). In the first iteration, a methylene substituted for the oxygen atom (69), which resulted in loss of inhibitory activity. However, deletion of the oxygen atom, generating a phenyl acetamide derivative, resulted in moderately potent inhibition (70, IC₅₀ = 120 nM). Interestingly, the stereochemical preference of the phenyl acetamide derivatives was the opposite of the ether derivatives. For example, the (S)-isomer 71 was inactive, while the (R)-isomer 72 displayed an IC₅₀ value of 22 nM. Another example was investigated to confirm this observation. The (R)-2,3-di-chlorophenyl derivative 22 was found to be a moderately potent CpIMPDH inhibitor, although it was less active than the (S)-ether 55.

Table 5.

SAR of benzyl amide derivatives of 1 for CpIMPDH inhibition.

ID	X	R1	IC50 (nM)
			(-) BSA	(+) BSA
69	Me	CH2Ph	> 5000	n.d.
70	Me	Ph	120 ± 20	130 ± 30
71	(S)-Me	Ph	>5000*	n.d.
72	(R)-Me	Ph	22.3 ± 5.9	28.0 ± 3.5
22	(R)-Me	2,3-di-ClPh		60 ± 3

Given the interesting results obtained with replacing the C-O bond of 1 with a C-C bond, the ether was exchanged with a secondary amine (Table 6). The 2,3-dichlorophenyl amine derivative 73 demonstrated slightly improved activity compared to the corresponding 2,3-dichlorophenyl ether 46. In addition, the (S)-isomer 15a proved to be a very potent CpIMPDH inhibitor (IC₅₀ = 0.5 nM), although the (S)-naphthyl amine derivative 15b was less active (IC₅₀ = 14 nM).

Table 6.

SAR of secondary amine derivatives of 1 for CpIMPDH inhibition.

ID	X	R1	IC50 (nM)
			(−) BSA	(+) BSA
73	Me	2,3-di-ClPh	2 ± 1	1.8 ± 0.5
15a	(S)- Me	2,3-di-ClPh	0.5 ± 0.1	1.1 ± 0.1
15b	(S)- Me	1-naphthyl	14 ± 6	15 ± 3

Evaluation of kinetic mechanism for CpIMPDH inhibition

The high throughput screen was designed to target the cofactor site since this site is the most diverged, and therefore most likely to yield inhibitors selective for the parasite enzyme.⁹ CpIMPDH, like other IMPDHs characterized to date, has a kinetic mechanism wherein substrates bind randomly and hydride transfer occurs forming a covalent E-XMP* intermediate and NADH.⁷ Products dissociate in an ordered fashion, with NADH release occurring before the hydrolysis and release of E-XMP*. In principle, IMPDH inhibitors that bind in the cofactor site can be competitive, uncompetitive or noncompetitive, depending on their relative affinities for the E, E•IMP and E-XMP* complexes. In practice, most such inhibitors are noncompetitive, suggesting comparable affinities for E•IMP and E-XMP*. Uncompetitive inhibition is also commonly observed, indicating a strong preference for E-XMP*. The inhibition mechanisms of four representative inhibitors (1, 63, 68 and 72) were evaluated. Surprisingly, the inhibition data with respect to NAD⁺ for all four compounds were best fit by competitive mechanism (Figure 3 and Table 7). However, the fit to a noncompetitve/mixed inhibition was not significantly inferior. This ambiguity is a consequence of NAD⁺ substrate inhibition, which prevents the use of saturating NAD⁺ concentrations.⁷ This observation suggests that these compounds have a strong preference for E•IMP. All four compounds are noncompetitive inhibitors with respect to IMP.

Figure 3.

The inhibitor binding site in IMPDH. (A) Structure of CpIMPDH in complex with IMP and 54 (violet-blue; PDB ID code 4IXH). (B) Structure of Chinese hamster IMPDH (gray; PDB ID code)¹⁹ in complex with IMP and MPA. (C) Overlay of C. parvum and Chinese hamster active sites. IMP, 54, MPA and residues interacting with the inhibitors are shown as sticks. Numbers in parentheses in panel C indicate Chinese hamster labeling. Water molecules are shown as red spheres and hydrogen bonding interactions are shown as blue dashed lines. An apostrophe indicates a residue from the adjacent monomer.

Table 7.

Mechanism of inhibition of CpIMPDH by selected compounds.

Cmpd	Substrate	Mechanism	Kis (nM)
1	IMP	NC	58 ± 5
	NAD	C	33 ± 8
63a	IMP	NC	1.6 ± 0.7
	NAD	C	0.9 ± 0.3
68	IMP	NC	210 ± 10
	NAD	C	150 ± 30
72	IMP	NC	100 ± 20
	NAD	C	40 ± 10

^aQ26 was analyzed using tight binding treatment;

NC: noncompetitive; C: competitive.

Mouse liver microsomal stability

A selected set of the CpIMPDH inhibitors was evaluated for metabolic stability in mouse liver microsomes (Table 8). Compounds were incubated with microsomes at 37 °C in the presence and absence of NADPH. The percentage of compound remaining at various time points was determined and then the data were fit to a first-order decay model to determine half-life. Three ether derivatives (55, 62 and 40a) demonstrated poor stability in both the presence and absence of NADPH (t_½ ≤ 12 min), whereas 54 was moderately more stable (t_½ = 30 min). In the case of two phenyl acetamide derivatives, the unsubstituted inhibitor 72 proved more stable (t_½ = 43 min) compared to the 2,3-di-chlorophenyl inhibitor 22 (t_½ = 25 min) in the presence of NADPH. Both compounds were quite stable in the absence of NADPH. For the amine derivatives, the 2,3-di-chlorophenyl inhibitor 15a also displayed moderate stability in the presence of NADPH (t_½ = 44 min), whereas the naphthyl derivative 15b was less stable in the presence or absence of NADPH (t_½ = 18 min and 27 min, respectively).

Table 8.

NADPH-dependent mouse liver microsomal stability.

ID	Connection	R1	R2	X	Y	t1/2 (min)
						+ NADPH	− NADPH
54	5	1-naphthyl	4-Py	(S)-Me	O	30	--
55	5	2,3-di-ClPh	4-Py	(S)-Me	O	9.0	12
62	5	2,3-di-ClPh	5-thiazolyl	(S)-Me	O	7.0	11
72	5	Ph	4-Py	(R)-Me	---	43	130a
22	5	2,3-di-ClPh	4-Py	(R)-Me	---	25	130
15a	5	2,3-di-ClPh	4-Py	(S)-Me	NH	44	110
15b	5	1-naphthyl	4-Py	(S)-Me	NH	18	27
40a	6	2,3-di-ClPh	4-Py	(S)-Me	NH	10	9

^aEstimated from a single time point at 45 min

The structure of CpIMPDH in complex with 54 and IMP

The structure of CpIMPDH in complex with IMP and 54 was solved at 2.10 Å resolution using molecular replacement with the structure of apo CpIMPDH (PDB ID code 3FFS) as the search model (Table S1). The structure (Figure 3A) revealed that the inhibitor binds in the active site and interacts with residues from two adjacent subunits, similarly as previously observed in the structure of CpIMPDH with C64 (PDB ID code 3KHJ)^10b. This binding mode is significantly different compared to hIMPDH inhibitors and it likely accounts for the selectivity of the benzoxazole compounds. Inhibitors of mammalian IMPDH such as mycophenolic acid (MPA) bind in the nicotinamide subsite and interact directly with the purine ring of the substrate, IMP. In addition, the MPA interaction extends into the adenine subside. However, the interactions for MPA are limited to the residues within the same monomer (Figure 3B). In contrast, the naphthalene of 54 stacks against the purine ring of IMP, in a manner similar to the 2-thiazole of C64, with the remainder of 54 extending across the subunit interface into the pocket in the adjacent monomer. Unlike C64, the 2-(4-pyridyl)benzoxazole of 54 fills much of this cavity and the pyridine nitrogen of 54 participates in a water-mediated hydrogen bond with the L170 main chain amide nitrogen (Figures 3A and 4). This hydrogen bond can account for the requirement of the 4-pyridyl or 5-thiazolyl substituents. These additional interactions are likely to explain the increased affinity of 54 relative to C64 (IC₅₀ = 28 nM and 6 nM for C64 and 54, respectively). The structure also reveals that the (S)-methyl group of 54 forms a hydrophobic interaction with M308. Inverting this center to the (R)-isomer would likely introduce a steric clash with E329, which forms a hydrogen bond with the amide NH of 54 and along with S354′ (where ′ denotes a residue from the adjacent subunit) and T221 forms a hydrogen bonding network with Y358′ (Figures 3A and 4). These observations provide a rationale for the observed stereochemical preference with regard to the substituent on the amide α-position in the ether and amine derivatives. In addition, the benzo-portion of the benzoxazole ring of 54 appears to interact with the side-chain of Y358′.

Figure 4.

Overlay of CpIMPDH structures with 54 (violet-blue; PDB ID code 4IXH) and C64 (PDB ID code 3KHJ).^10b IMP, 54, C64 and the interaction residues are shown as sticks. A water molecule is shown as a red sphere and hydrogen bonding interactions are shown as blue dashed lines. An apostrophe denotes a residue from the adjacent monomer.

Evaluation of antiparasitic activity

Although the generation of potent CpIMPDH inhibitors has been accomplished with several structurally distinct compound classes, achieving antiparasitic activity in C. parvum remains a challenge. This organism cannot be continuously cultured in vitro, so such assays are poor mimics of in vivo infection in addition to having a poor dynamic range. However, the related intracellular parasite T. gondii has proven to be a well behaved organism that can be engineered to express fluorescent markers, facilitating its use in screening.²⁰ Previously, we genetically engineered a T. gondii strain that relies on CpIMPDH (Toxo/CpIMPDH) to synthesize guanine nucleotides.¹⁸ In contrast, the wild type T. gondii strain RH (Toxo/WT) contains a eukaryotic IMPDH that is resistant to CpIMPDH inhibitors, thus providing target validation as well as a measure of host cell cytotoxicity¹⁸.

A set of twenty-two CpIMPDH inhibitors were evaluated for activity in both Toxo/CpIMPDH and Toxo/WT assays (Table 9). Four compounds (46, 48, 40a and 73) demonstrated EC₅₀ values ≤ 250 nM in the Toxo/CpIMPDH assay and selectivity > 30-fold versus Toxo/WT. Thus, the 2,3-dichlorophenyl or 1-naphthyl ethers or amines at either the 5- or -6-positions of the 2-(4-pyridyl)- or 2-(thiazolyl)benzoxazoles translated into encouraging antiparasitic activity. Furthermore, two of these compounds (40a and 73) demonstrated EC₅₀ values ≤ 30 nM and selectivity > 150-fold, indicating that the 2,3-dichlorophenyl ether or amine at either the 5- or 6-positions of the 2-(4-pyridyl)benzoxazole might be preferable. Based on the in vitro and cellular properties, these compounds are candidates for evaluation in an animal model of cryptosporidiosis.

Table 9.

Antiparasitic activity of select compounds. Assays as described in Methods¹⁸. Unless otherwise stated all values are the average of three independent determinations.

Compound	EC50 (μM)		Selectivitya
	Toxo/WT	Toxo/CpIMPDH
1	1.3 ± 0.1 b	0.65 ± 0.03 b	2
26	9 ± 3	9 ± 3	1
40a	3 ± 1	0.02 ± 0.02	150
41	3.0 ± 1.0	0.9 ± 0.4	3
43	7 ± 4	4 ± 2	2
44	>25	0.5 ± 0.2	>50
45	1.1 ± 0.4	0.5 ± 0.1	2
46	8 ± 2	0.22 ± 0.04	40
48	2.4 ± 0.3	0.20 ± 0.09 b	34
54	2.2 ± 0.6	0.4 ± 0.3	5
55	5 ± 2	0.3 ± 0.1	16
56	1.7 ± 0.6	0.8 ± 0.4	2
57	3 ± 1	0.19 ± 0.03	14
58	21 ± 6	0.7 ± 0.1	30
62	3.3 ± 0.5	0.2 ± 0.01	15
63	3.2 ± 0.8	0.30 ± 0.3	11
64	5 ± 4	2 ± 2	3
65	2.2 ± 0.8	0.3 ± 0.1	7
66	1.7 ± 0.7	1.0 ± 0.1	1.7
70	5 ± 3	2.5 ± 0.2	1.9
72	2.1 ± 0.5	0.4 ± 0.1	5
73	4 ± 2	0.02 ± 0.02	200

^aSelectivity is the ratio of EC₅₀ Toxo/CpIMPDH to EC₅₀ Toxo/WT.

^bTwo determinations.

Evaluation of mammalian cytotoxicity activity

A subset of compounds (15a, 40a, 41, 44, 46 and 57) were also evaluated for cytotoxicity against four mammalian cell lines (HeLa, HEK293, COS and CHO). Viability was determined by monitoring metabolic activity with alamarBlue^® assay. None of the compounds displayed significant toxicity (LD₅₀ > 50 μM) against all four cell lines, except 41 which exhibited an LD₅₀ ~ 12.5 μM in HEK293 cells.

Conclusions

A SAR study of CpIMPDH inhibitor 1 revealed that 2,3-di-substituted phenyl ethers improved potency, while a 4-pyridyl or 5-thiazolyl group was necessary at the 2-position of the benzoxazole. In addition, the (S)-isomers were significantly more potent compared to the corresponding (R)-isomers, but the connectivity of the amide could be transposed from the 5-position of the benzoxazole to the 6-position. The ether oxygen atom could be deleted generating α-arylamides where the chiral preference switched with the (R)-isomer being more potent. Furthermore, the ether oxygen atom could be replaced with an NH without jeopardizing potent CpIMPDH inhibitory activity and again the (S)-isomer was preferred. The secondary amine derivative 15a (Q67) or its racemic version 73 demonstrated excellent CpIMPDH inhibitory activity in the presence or absence of BSA and moderate stability in the presence of mouse liver microsomes, as well as superb activity and selectivity in a T. gondii surrogate cell assay of C. parvum infection and no significant cytotoxicity against a panel of four mammalian cells. Finally, a co-crystal structure of CpIMPDH with IMP and 54 (Q21) allowed for a more complete understanding of the structure-activity relationships observed with the benzoxazole inhibitors and also demonstrated that this compound series adopts a similar binding mode as a structurally distinct and previously reported CpIMPDH inhibitor C64.^10b This information should assist in the continuing development of CpIMPDH inhibitors that will allow for further understanding of this apicomplexan parasite and its host interactions, advancing the treatment of cryptosporidiosis.

Experimental Section

Chemistry Material and Methods

Unless otherwise noted, all reagents and solvents were purchased from commercial sources and used without further purification. Compounds 1, 50, 51, 60 and 61 were purchased from Chembridge Corporation (San Diego, CA 92121, USA). All reactions were performed under a nitrogen atmosphere in dried glassware unless otherwise noted. All NMR spectra were obtained using a 400 MHz spectrometer and conducted in CDCl₃, unless otherwise indicated. For ¹H NMR, all chemical shifts are reported in δ units ppm and are referenced to tetramethylsilane (TMS). All chemical shift values are also reported with multiplicity, coupling constants and proton count. Likewise, for ¹³C NMR, all chemical shifts are reported in δ units ppm and are referenced to the central line of the triplet at 77.23 ppm for those conducted in CDCl₃. Coupling constants (J values) are reported in hertz. Column chromatography was carried out on SILICYCLE SiliaFlash silica gel F60 (40–63 μm, mesh 230–400). High-resolution mass spectra (HRMS) were obtained using a Q-tof UE521 mass spectrometer (University of Illinois, SCS, and Mass Spectrometry Lab).

HPLC conditions

All final compounds have a chemical purity >98% as determined by analysis using a Agilent 1100 HPLC instrument equipped with a quaternary pump and a Zorbax® SB-C8 column (30 × 4.6 mm, 3.5 mm). UV absorption was monitored at 254 nm. The injection volume was 5 μL. HPLC gradient was 5 % acetonitrile and 95 % water (both solvents contain 0.1% trifluoroacetic acid) with a total run time of 2.5 min and a flow rate of 3.0 mL/min.

Enantiomeric purity was determined using HPLC analysis on a Agilent 1100 Series instrument equipped with a quaternary pump using a Chiralpak AS-H Column (250 × 4.6 mm) for all chiral compounds unless noted. The UV absorption was monitored at 254 nm and the injection volume was 10 μL. A Chiralpak OD-H column was used for 22, 40a, 40b and 58, and an OJ-H column was used for 15b, 73 and 74. For these latter two columns the UV absorption was monitored at 220 nm and an injection volume 20 μL was used. HPLC gradient for all compounds was 70 % n-hexane and 30 % i-propanol and a flow rate of 1.0 mL/min.

General procedure for the preparation of 5-nitro-2-arylbenzo[d]oxazoles (3). Exemplified for 5-nitro-2-(pyridine-4-yl)benzo[d]oxazole (3, R₁ = 4-Py, X = H)

To a solution of 2-amino-4-nitrophenol (500 mg, 3.24 mmol) in anhydrous xylene (10 mL) in a 100 mL three-neck flask under an oxygen atmosphere was added 4-pyridine carboxaldehyde (347 mg, 3.23 mmol) and Darco KB (600 mg). The solution was stirred at 120 °C for 4 h. The reaction mixture was allowed to cool to room temperature and then filtered with the aid of Celite. The filtrate was concentrated and the residue was purified by silica gel column chromatography using a mixture of ethyl acetate/n-hexane (50:50) to give 5-nitro-2-(pyridine-4-yl)benzo[d]oxazole (600 mg, 77%) as a yellow solid. ¹H NMR (CDCl₃, 400 MHz) δ 7.76 (d, J = 9.2 Hz, 1H), 8.11 (d, J = 6 Hz, 2H), 8.40 (d, J₁ = 12 Hz, J₂ = 2.4 Hz, 1H), 8.73 (d, J = 2 Hz, 1H), 8.89 (d, J = 6 Hz, 2H).

General procedure for the preparation of 5-amino-2-aryl-benzo[d]oxazoles (4). Exemplified for 5-amino-2-(pyridine-4-yl)benzo[d]oxazole (4, R₁ = 4-Py, X = H)

To a solution of 5-nitro-2-(pyridine-4-yl)benzo[d]oxazole (600 mg, 2.48 mmol) in ethyl acetate/MeOH (1:1, 10 mL) was added a catalytic amount of 10% Pd-C. The reaction mixture was placed under a hydrogen atmosphere and stirred for 6 h at room temperature. The mixture was then filtered, concentrated and the residue purified by flash column chromatography on silica gel using 100% ethyl acetate as eluent to afford 5-amino-2-(pyridine-4-yl)benzo[d]oxazole (450 mg, 86%) as a yellow solid. ¹H NMR (CDCl₃, 400 MHz) δ 6.78 (dd, J₁ = 12 Hz, J₂ = 2.4 Hz, 1H), 7.07 (d, J = 2 Hz, 1H), 7.40 (d, J = 4.8 Hz, 1H), 8.05 (dd, J₁ = 8 Hz, J₂ = 1.6 Hz, 2H), 8.79 (dd, J₁ = 4 Hz, J₂ = 1.6 Hz, 2H), 3.78 (s, 2H).

General procedure for the preparation of (±)-7. Exemplified for 2-(2,3-dichlorophenoxy)propionic acid (7, R₂ = 2,3-di-ClPh, R₃ = Me, Y = O)

To a solution of 2,3-dichlorophenol (200 mg, 1.22mmol) in anhydrous DMF (15 mL) was added K₂CO₃ (505 mg, 3.66 mmol) and ethyl 2-bromopropionate (287.1 mg, 1.58 mmol). The reaction mixture was stirred at room temperature for 5 h under a nitrogen atmosphere, diluted with water (50 mL) and extracted with ethyl acetate (3 X 50 mL). Note, when Y = NH this reaction was conducted at 70 °C for 3 h. The organic extracts were combined, washed with brine, dried over anhydrous MgSO₄, filtered and concentrated in vacuo. The residue was purified by silica gel column chromatography using a mixture of ethyl acetate and n-hexane (10:90) to give ethyl 2-(2,3-dichlorophenoxy)propionate (290 mg, 1.2 mmol, 90%) as a colorless liquid. The ester (290 mg, 1.60 mmol) was dissolved in THF: H₂O (2:1), and then NaOH (132 mg, 3.3 mmol) was added. The reaction mixture was heated at 80 °C for 3 h. After the reaction mixture was allowed to cool to room temperature, 1N HCl was added until a pH of 7 was reached and then the mixture was extracted with DCM. The organic layers were dried over anhydrous MgSO₄, filtered, and concentrated. The residue was purified by silica gel column chromatography using a mixture of ethyl acetate/n-hexane (60:40) to afford 2-(2,3-dichlorophenoxy)propionic acid (270 mg, 95%) as a white solid.

General procedure for the preparation of (R)- and (S)-7. Exemplified for (S)-2-(2,3-dichlorophenoxy)propionic acid (7, R₂ = 2,3-di-ClPh, R₃ = (S)-Me, Y = O)

2,3-Dichlorophenol (150 mg, 0.92 mmol) was dissolved in anhydrous DCM (6 mL) under a nitrogen atmosphere and then methyl (R)-(+)-2-(4-hydroxyphenoxy)propionate (143.7 mg, 1.38 mmol) was added. After the reaction mixture was cooled to 0 °C, PPh₃ (289.4 mg, 1.10 mmol) was added portion-wise and the reaction mixture was stirred for 10 minutes. Then DEAD (240 mg, 1.37 mmol) was slowly added. The reaction mixture was stirred for 12 h at room temperature and then 10 mL of water was added and the mixture extracted with DCM. The organic layers were washed with brine, dried over anhydrous MgSO_4, filtered and concentrated. The residue was purified by column chromatography on silica gel using ethyl acetate/n-hexane (10:90) to yield methyl (S)-2-(2,3-dichlorophenoxy)propionate (180 mg, 0.76 mmol, 83%) as a colorless liquid. The ester (180 mg, 0.72 mmol) was added to a mixture of 6 mL THF in 3N HCl (2:8), and then heated at 70 °C for 6 h. The reaction mixture was allowed to cool to room temperature then extracted with DCM. Organic layers were washed with brine, dried over anhydrous MgSO_4, filtered, and concentrated. The residue was purified by silica gel column chromatography using ethyl acetate/n-hexane (60:40) to afford (S)-2-(2,3-dichlorophenoxy)propionic acid (110 mg, 62%) as a white solid. ¹H NMR (CDCl₃, 400 MHz) δ 1.51(d, J = 7.2 Hz, 3H), 4.27 (q, J = 7.2 Hz, 1H), 7.16 – 7.24 (m, 2H), 7.37 (dd, J₁ = 3.6 Hz, J₂ = 1.6 Hz, 1H).

General procedure for the preparation of benzoxazoles (8). Exemplified for (S)-2-(2,3-dichlorophenoxy)-N-(2-(pyridin-4-yl)benzo[d]oxazol-5-yl)propionamide (55)

To a solution of 5-amino-2-(pyridine-4-yl)benzo[d]oxazole (266.7 mg, 1.27 mmol) and (S)-2-(2,3-dichlorophenoxy)propionic acid (300 mg, 1.27 mmol) were dissolved in anhydrous DMF (5 mL) under a nitrogen atmosphere. The reaction mixture was cooled to 0 °C and then EDCI•HCl (489.6 mg, 2.55 mmol) was added. The reaction mixture was stirred for 12 h at room temperature. Volatiles were removed under reduced pressure and the mixture was dissolved in 20 mL DCM. The organic layer was washed sequentially with a saturated aqueous solution of sodium bicarbonate and brine. The organic layers were dried over anhydrous MgSO₄, filtered, and concentrated. The residue was purified by silica gel column chromatography using ethyl acetate/n-hexane (50:50) to give 55 (390 mg, 72%) as a white solid. ¹H NMR (CDCl₃, 400 MHz) δ 1.76 (d, J = 6.8 Hz, 3H), 4.90 (q, J = 6.8 Hz, 1H), 6.93 (dd, J₁ = 7.2 Hz, J₂ = 2.4 Hz, 1H), 7.17 – 7.23 (m, 2H), 7.56 – 7.61 (m, 2H), 8.06 (dd, J₁ = 4 Hz, J₂ = 1.6 Hz, 2H), 8.17 (s, 1H), 8.81 – 8.83 (m, 3H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.6, 76.9, 111.2, 112.1, 113.2, 119.2, 121.2, 122.9, 124.3, 128.1, 134.3, 134.5, 134.7, 142.4, 148.0, 150.9, 153.7, 161.8, 169.2; ESIHRMS for C₂₁H₁₆N₃O₃Cl₂ (M+H)⁺ calcd. 428.0569, found 428.0568, Chiral purity (% ee > 98, t_R=12.84 min).

The following compounds were prepared in a similar manner:

2-(2-chlorophenoxy)-N-(2-(pyridine-4-yl)benzo[d]oxazol-5-yl)propionamide (41)

71%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.77 (d, J = 6.4 Hz, 3H), 4.90 (q, J = 6.8 Hz, 1H), 7.02 (t, J = 7.6 Hz, 2H), 7.25 – 7.29 (m, 1H), 7.45 (d, J = 8 Hz, 1H), 7.57 (d, J = 8.4 Hz, 1H), 7.64 (dd, J₁ = 8 Hz, J₂ = 2 Hz, 1H), 8.06 (d, J = 5.2 Hz, 2H), 8.17 (s, 1H), 8.82 (d, J = 4.4 Hz, 2H), 8.95 (s, 1H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.7, 76.8, 111.1, 111.9, 115.6, 119.2, 121.1, 123.5, 123.8, 128.4, 130.8, 134.3, 134.9, 142.3, 147.9, 150.9, 152.4, 161.7, 169.5; ESI-HRMS for C₂₁H₁₇N₃O₃Cl (M+H)⁺ calcd. 394.0958 found 394.0961.

2-(4-chlorophenoxy)-N-(2-(pyridine-4-yl)benzo[d]oxazol-5-yl)propionamide (42)

72%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.69 (d, J = 6.8 Hz, 3H), 4.79 (q, J = 6.8 Hz, 1H), 6.94 (d, J = 8.8 Hz, 2H), 7.30 (d, J = 8.4 Hz, 2H), 7.51 – 7.58 (m, 2H), 8.08 (dd, J₁ = 10 Hz, J₂ = 2 Hz, 3H), 8.29 (s, 1H), 8.82 (d = 4.8 Hz, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.8, 76.0, 111.2, 112.5, 117.3, 119.5, 121.2, 127.8, 130.1, 134.3, 134.5, 142.4, 148.1, 150.9, 155.3, 161.8, 170.1; ESI-HRMS for C₂₁H₁₇N₃O₃Cl (M+H)⁺ calcd. 394.0958 found 394.0961.

2-phenoxy-N-(2-(pyridine-4-yl)benzo[d]oxazol-5-yl)propionamide (43)

63%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.69 (d, J = 6.4 Hz, 3H), 4.84 (q, J = 6.4 Hz, 1H), 7.07 – 6.99 (m, 3H), 7.34 (t, J = 8 Hz, 2H), 7.54 (t, J = 8.4 Hz, 2H), 8.08 (d, J = 13.6 Hz, 3H), 8.42 (s, 1H), 8.82 (s, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.9, 75.6, 111.1, 112.4, 115.9, 119.6, 120.5, 121.2, 121.2, 121.3, 122.7, 128.1, 130.1, 134.3, 134.6, 142.4, 148.0, 150.9, 156.8, 161.7, 170.6; ESI-HRMS for C₂₁H₁₈N₃O₃ (M+H)⁺ calcd. 360.1348, found 360.1350.

2-(4-methoxyphenoxy)-N-(2-(pyridine-4-yl)benzo[d]oxazol-5-yl)propionamide (44)

73%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.66 (d, J = 6.8 Hz, 3H), 3.78 (s, 3H), 4.72 (q, J = 6.8 Hz, 1H), 6.86 (d, J = 9.2 Hz, 2H), 6.94 (d, J = 9.2 Hz, 2H), 7.56 (s, 2H), 8.07 (d, J = 6 Hz, 2H), 8.11 (s, 1H), 8.44 (s, 1H), 8.81 (d, J = 6 Hz, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.8, 55.8, 76.6, 94.6, 111.1, 112.3, 115.1, 117.4, 119.5, 121.2, 134.3, 134.7, 142.4, 148.0, 150.9, 155.2, 161.7, 170.8; ESI-HRMS for C₂₂H₂₀N₃O₄ (M+H)⁺ calcd. 390.1454 found 390.1457.

2-(3-chlorophenoxy)-N-(2-(pyridine-4-yl)benzo[d]oxazol-5-yl)propionamide (45)

73%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.68 (d, J = 6.8 Hz, 3H), 4.82 (q, J = 6.8 Hz, 1H), 6.86 – 6.89 (m, 1H), 7.02 (d, J = 7.6 Hz, 2H), 7.23 – 7.27 (m, 1H), 7.51 – 7.57 (m, 2H), 8.05 (d, J = 5.2 Hz, 2H), 8.09 (s, 1H), 8.37 (s, 1H), 8.81 (s, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.7, 75.8, 111.1, 112.5, 113.8, 116.8, 119.6, 121.2, 123.0, 130.9, 134.2, 134.5, 135.5, 142.4, 148.1, 150.9, 157.4, 161.7, 170.0; ESIHRMS for C₂₁H₁₇N₃O₃Cl (M+H)⁺ calcd. 394.0958, found 394.0959.

2-(2,3-dichlorophenoxy)-N-(2-(pyridine-4-yl)benzo[d]oxazol-5-yl)propionamide (46)

71%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.77 (d, J = 6.8 Hz, 3H), 4.91 (q, J = 6.8 Hz, 1H), 6.93 (dd, J₁ = 6.6 Hz, J₂ = 2.8 Hz, 1H), 7.20 – 7.23 (m, 2H), 7.60 (d, J = 1.2 Hz, 2H), 8.08 (d, J = 4.4 Hz, 2H), 8.18 (s, 1H), 8.83 (s, 3H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.3, 76.6, 111.0, 111.8, 113.0, 119.0, 120.9, 122.7, 124.1, 127.8, 134.1, 134.3, 134.5, 142.2, 147.8, 150.7, 153.5, 161.5, 168.9; ESI-HRMS for C₂₁H₁₆N₃O₃Cl₂ (M+H)₊ calcd. 428.0569, found 428.0569.

2-(2,6-dichlorophenoxy)-N-(2-(pyridine-4-yl)benzo[d]oxazol-5-yl)propionamide (47)

67%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.61 (d, J = 6.8 Hz, 3H), 5.11 (q, J = 6.8 Hz, 1H), 7.09 (t, J = 7.6 Hz, 1H), 7.37 (d, J = 8.4 Hz, 2H), 7.60 (d, J = 8.8 Hz, 1H), 7.18 (dd, J₁ = 8.8 Hz, J₂ = 1.6 Hz, 1H), 8.08 (d, J = 5.2 Hz, 2H), 8.20 (s, 1H), 8.82 (d, J = 5.2 Hz, 2H), 8.99 (s, 1H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.2, 79.5, 111.1, 112.1, 119.4, 121.2, 126.1, 129.5, 129.6, 134.3, 135.0, 142.4, 148.0, 148.7, 150.9, 169.5, 195.9; ESI-HRMS for C₂₁H₁₆N₃O₃Cl₂ (M+H)⁺ calcd. 428.0569, found 428.0567.

2-(naphthalene-1-yloxy)-N-(2-(pyridine-4-yl)benzo[d]oxazol-5-yl)propionamide (48)

73%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.84 (d, J = 6.4 Hz, 3H), 5.04 (q, J = 6.8 Hz, 1H), 6.92 (d, J = 7.6 Hz, 1H), 7.39 (t, J = 8 Hz, 1H), 7.61 – 7.48 (m, 5H), 7.89 – 7.86 (m,1H), 8.05 – 8.08 (m, 3H), 8.33 – 8.36 (m, 1H), 8.41 (s, 1H), 8.80 (s, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 19.1, 76.2, 107.3, 111.1, 112.4, 119.6, 121.21, 121.25, 121.5, 122.4, 125.8, 126.0, 126.1, 127.0, 128.1, 134.2, 134.6, 134.9, 142.4, 148.0, 150.9, 152.6, 170.6; ESI-HRMS for C₂₅H₂₀N₃O₃ (M+H)⁺ calcd. 410.1505, found 410.1508.

2-(4-chloronaphthalen-1-yloxy)-N-(2-(pyridin-4-yl)benzo[d]oxazol-5-yl)propionamide (49)

68%; white solid. ¹H NMR (CDCl₃, 400 MHz) δ 1.83 (d, J = 6.8 Hz, 3H), 5.0 (q, J = 6.8 Hz, 1H), 6.84 (d, J = 8.4 Hz, 1H), 7.46 – 7.50 (m, 2H), 7.54 (d, J = 8.8 Hz, 1H), 7.63 – 7.71 (m, 2H), 8.04 – 8.07 (m, 3H), 8.27 (d, J = 8 Hz, 1H), 8.36 (d, J = 8 Hz, 2H), 8.81 (d, J = 4 Hz, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 19.0, 76.5, 107.3, 111.1, 112.5, 119.6, 121.1, 122.0, 124.9, 125.5, 125.9, 126.9, 128.1, 131.8, 134.2, 134.4, 134.4, 134.7, 142.4, 148.1, 150.9, 151.7, 161.8, 170.3; ESI-HRMS for C₂₅H₁₉N₃O₃Cl (M+H)⁺ calcd. 444.1115, found 444.1119.

3-methyl-2-(naphthalene-1-yloxy)-N-(2-(pyridine-4-yl)benzo[d]oxazol-5-yl)butanamide (52)

67%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.18 (d, J = 6.8 Hz, 3H), 1.25 (d, J = 6.8 Hz, 3H), 2.51 – 2.59 (m, 1H), 4.69 (d, J = 4 Hz, 1H), 6.84 (d, J = 7.6 Hz, 1H), 7.31 (t, J = 8 Hz, 1H), 7.37 – 7.47 (m, 3H), 7.51 – 7.54 (m, 2H), 7.80 (t, J = 6.4 Hz, 1H), 7.95 – 7.97 (m, 3H), 8.34 (d, J = 5.6 Hz, 2H), 8.72 (d, J = 5.2 Hz, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 17.5, 19.6, 32.5, 84.5, 106.7, 111.0, 112.7, 119.9, 121.1, 121.5, 122.1, 125.7, 126.0, 127.0, 128.1, 134.2, 134.4, 134.8, 142.2, 148.0, 150.7, 153.5, 161.6, 169.8; ESI-HRMS for C₂₇H₂₄N₃O₃ (M+H)⁺ calcd. 438.1818, found 438.1822.

(R)-2-(naphthalene-1-yloxy))-N-(2-(pyridine-4-yl)benzo[d]oxazol-5-yl)propionamide (53)

76%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.83 (d, J = 6.8 Hz, 3H), 5.03 (q, J = 6.8 Hz, 1H), 6.91 (d, J = 7.6 Hz, 1H), 7.38 (t, J = 8 Hz, 1H), 7.50 – 7.58 (m, 5H), 7.85 – 7.87 (m, 1H), 8.03 – 8.07 (m, 3H), 8.34 (t, J = 2.8 Hz, 1H), 8.47 (s, 1H), 8.80 (bs, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 19.1, 67.3, 76.2, 107.3, 111.1, 112.4, 119.6, 121.2, 121.5, 122.4, 125.8, 126.0, 126.1, 127.0, 128.1, 134.3, 134.6, 134.9, 142.3, 148.0, 150.9, 152.6, 161.7, 170.6; ESI-HRMS for C₂₅H₂₀N₃O₃ (M+H)⁺ calcd. 410.1505, found 410.1508, Chiral purity (% ee 98.1, t_R=22.87).

(S)-2-(naphthalene-1-yloxy))-N-(2-(pyridine-4-yl)benzo[d]oxazol-5-yl)propionamide (54)

71%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.78 (d, J = 6.8 Hz, 3H), 4.96 (q, J = 6.8 Hz, 1H), 6.85 (d, J = 7.6 Hz, 1H), 7.30 (t, J = 8 Hz, 1H), 7.50 – 7.42 (m, 5H), 7.77 – 7.79 (m, 1H), 7.92 (dd, J₁ = 4.4 Hz, J₂ = 1.6 Hz, 2H), 8.02 (s, 1H), 8.26 – 8.23 (m, 1H), 8.60 (s, 1H), 8.70 (d, J = 4.4 Hz, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 19.1, 76.2, 107.2, 111.0, 112.5, 119.7, 121.1, 121.5, 122.3, 125.0, 125.8, 126.0, 126.9, 128.0, 134.1, 134.7, 134.8, 142.2, 147.9, 150.8, 152.7, 161.6, 170.7; ESI-HRMS for C₂₅H₂₀N₃O₃ (M+H)⁺ calcd. 410.1505, found 410.1507, Chiral purity (% ee 98.9, t_R=12.08).

(S)-2-(2-chloro-3-(trifluoromethyl)phenoxy)-N-(2-(pyridine-4-yl)benzo[d]oxazol-5-yl)propionamide (56)

70%; cream colored solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.78 (d, J = 6.8 Hz, 3H), 4.94 (q, J = 6.4 Hz, 1H), 7.21 (d, J = 7.6 Hz, 1H), 7.37 – 7.44 (m, 2H), 7.59 (s, 2H), 8.07 (d, J = 5.2 Hz 2H), 8.19 (s, 1H), 8.82 (d, J = 5.6 Hz, 3H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.5, 76.9, 111.2, 112.1, 118.4, 119.2, 121.1, 122.3, 124.0, 130.4, 134.2, 134.6, 142.4, 148.0, 150.9, 153.5, 161.8, 168.9; ESI-HRMS for C₂₂H₁₆N₃O₃ClF₃ (M+H)⁺ calcd. 462.0832, found 462.0834, Chiral purity (% ee 99.4, t_R=11.83).

(S)-2-(2-chloro-3-nitrophenoxy)-N-(2-(pyridine-4-yl)benzo[d]oxazol-5-yl)propionamide (57)

77%; cream colored solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.80 (d, J = 6.8 Hz, 3H), 4.97 (q, J = 6.8 Hz, 1H), 7.24 (d, J = 8.4 Hz, 1H), 7.44 (t, J = 8.4 Hz, 1H), 7.53 – 7.62 (m, 3H), 8.07 – 8.09 (m, 2H), 8.20 (s, 1H), 8.68 (s, 1H), 8.82 – 8.84 9m, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.5, 76.9, 111.3, 112.2, 117.2, 118.0, 118.7, 119.3, 121.2, 128.4, 134.2, 134.4, 142.4, 148.1, 149.9, 150.9, 153.7, 161.9, 168.6; ESI-HRMS for C₂₁H₁₆N₄O₅Cl (M+H)⁺ calcd. 439.0809, found 439.0805, Chiral purity (% ee >99, t_R=35.93).

(S)-2-(2,3-dimethoxyphenoxy)-N-(2-(pyridine-4-yl)benzo[d]oxazol-5-yl)propionamide (58)

79%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.78 (d, J = 6.8 Hz, 3H), 3.89 (s, 3H), 4.01 (s, 3H), 4.78 (q, J = 6.8 Hz, 1H), 6.67 – 6.71 (m, 2H), 7.04 (t, J = 5.6 Hz, 1H), 7.55 (d, J = 8.8 Hz, 1H), 7.68 (dd, J₁ = 9.2 Hz, J₂ = 2.4 Hz, 1H), 8.07 (dd, J₁ = 4.4 Hz, J₂ = 1.6 Hz, 2H), 8.20 (d, J = 2 Hz, 1H), 8.82 (dd, J₁ = 4.4 Hz, J₂ = 1.2 Hz, 2H), 9.58 (s, 1H); ¹³C NMR (CDCl₃, 100 MHz) δ 20.5, 57.0, 62.5, 80.3, 108.3, 111.4, 111.8, 112.7, 120.1, 122.0, 125.7, 135.3, 136.4, 140.4, 143.1, 148.6, 151.7, 152.8, 154.9, 162.4, 171.4; ESI-HRMS for C₂₃H₂₂N₃O₅ (M+H)⁺ calcd. 420.1559, found 420.1557, Chiral purity (% ee >99, t_R=17.41).

2-(2,4-dichlorophenoxy)-N-(2-(phenylbenzo[d]oxazol-5-yl)propionamide (59)

73%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.74 (d, J = 6.8 Hz, 3H), 4.84 (q, J = 6.8 Hz, 1H), 6.94 (d, J = 8.8 Hz, 1H), 7.23 (dd, J₁ = 8.8 Hz, J₂ = 2.4 Hz, 1H), 7.44 (d, J = 2.4 Hz, 1H), 7.54 – 7.51 (m,4H), 7.56 (d, J = 2 Hz, 1H), 7.59 (d, J = 2 Hz, 1H), 8.05 (d, J = 1.6 Hz, 1H), 8.24 (dd, J = 8 Hz, 2H), 8.75 (s, 1H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.6, 76.9, 110.8, 111.6, 116.4, 118.0, 124.7, 127.1, 127.8, 128.0, 128.4, 129.1, 130.6, 131.9, 134.2, 142.8, 148.0, 151.3, 164.2, 169.1; ESI-HRMS for C₂₂H₁₇N₂O₃Cl₂ (M+H)⁺ calcd. 427.0616 found 427.0619.

(S)-2-(2,3-dichlorophenoxy)-N-(2-(thiazol-5-yl)benzo[d]oxazol-5-yl)propionamide (62)

76%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.76 (d, J = 6.8 Hz, 3H), 4.89 (q, J = 6.8 Hz, 1H), 6.92 (dd, J₁ = 7.2 Hz, J₂ = 2.4 Hz, 1H), 7.17 – 7.22 (m, 2H), 7.55 (q, J = 8.8 Hz, 2H), 8.09 (s, 1H), 8.64 (s, 1H), 8.81 (s, 1H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.6, 76.9, 110.9, 111.6, 113.2, 118.6, 120.5, 122.9, 124.3, 125.7, 128.1, 134.7, 142.4, 145.7, 147.6, 153.7, 156.4, 158.0, 169.1; ESI-HRMS for C₁₉H₁₄N₃O₃SCl₂ (M+H)⁺ calcd. 434.0133, found 434.0136, Chiral purity (% ee 99.1, t_R=15.5).

(S)-2-(naphthalene-1-yloxy)-N-(2-(thiazol-5-yl)benzo[d]oxazol-5-yl)propionamide (63)

78%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.81 (d, J = 6.8 Hz, 3H), 5.01 (q, J = 6.4 Hz, 1H), 6.89 (d, J = 7.6 Hz, 1H), 7.35 (t, J = 7.6 Hz, 1H), 7.45 – 7.54 (m, 5H), 7.82 (t, J = 4.8 Hz, 1H), 7.96 (s, 1H), 8.29 (d, J = 5.2 Hz, 1H), 8.55 (d, J = 21.6 Hz, 2H), 8.91 (s, 1H); ¹³C NMR (CDCl₃, 100 MHz) δ 19.1, 76.2, 107.3, 110.7, 112.0, 119.0, 121.5, 122.3, 125.6, 125.8, 126.0, 126.1, 127.0, 128.0, 134.6, 134.8, 142.2, 145.6, 147.6, 152.6, 156.4, 157.8, 170.6; ESI-HRMS for C₂₃H₁₈N₃O₃S (M+H)⁺ calcd. 416.1069, found 416.1075, Chiral purity (% ee 98.7, t_R=12.53).

2-(4-chloronaphthalen-1-yloxy)-N-(2-(thiazol-5-yl)benzo[d]oxazol-5-yl)propionamide (64)

64%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.82 (d, J = 6.8 Hz, 3H), 4.99 (q, J = 6.8 Hz, 1H), 6.83 (d, J = 8.4 Hz, 1H), 7.44 – 7.50 (m, 3H), 7.62 – 7.71 (m, 2H), 7.98 (d, J = 2 Hz, 1H), 8.25 – 8.37 (m, 3H), 8.62 (s, 1H), 8.96 (s, 1H); ¹³C NMR (CDCl3, 100 MHz) δ 19.1, 76.6, 107.4, 110.9, 112.1, 119.0, 122.0, 125.0, 125.6, 126.0, 126.9, 127.0, 128.2, 131.8, 134.5, 142.4, 145.8, 147.8, 151.7, 156.5, 161.8, 166.4, 170.3; ESI-HRMS for C₂₃H₁₇N₃O₃SCl (M+H)⁺ calcd. 450.0679, found 450.0680.

(S)-2-(2,3-dichlorophenoxy)-N-(2-(thiazol-2-yl)benzo[d]oxazol-5-yl)propionamide (65)

69%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.72 (d, J = 6.8 Hz, 3H), 4.85 (q, J = 6.8 Hz, 1H), 6.89 (dd, J₁ = 8 Hz, J₂ = 2 Hz, 1H), 7.12 – 7.18 (m, 2H), 7.55 (bs, 2H), 7.59 (d, J = 2.8 Hz, 1H), 8.03 (d, J = 3.2 Hz, 1H), 8.14 (d, J = 1.2 Hz, 1H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.6, 77.0, 111.5, 112.0, 113.2, 119.5, 122.9, 123.5, 124.3, 128.0, 134.5, 134.9, 142.0, 145.3, 147.8, 153.7, 154.7, 158.0, 169.2; ESI-HRMS for C₁₉H₁₄N₃O₃SCl₂ (M+H)⁺ calcd. 434.0133, found 434.0132, Chiral purity (% ee >99, t_R=12.88).

N-(2-(1H-pyrrol-2-yl)benzo[d]oxazol-5-yl)2-(naphthalene-1-yloxy)propionamide (66)

70%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.82 (d, J = 6.8 Hz, 3H), 5.02 (q, J = 6.4 Hz, 1H), 6.36 (dd, J₁ = 2.4 Hz, J₂ = 6 Hz, 1H), 6.91 (d, J = 8 Hz, 1H), 7.06 (d, J = 1.6 Hz, 2H), 7.32 – 7.43 (m,3H), 7.52 – 7.60 (m, 3H), 7.85 – 7.89 (m, 2H), 8.34 (d, J = 12 Hz, 2H), 9.87 (d, J = 1.6 Hz, 1H); ¹³C NMR (CDCl₃, 100 MHz) δ 19.1, 76.2, 107.3, 110.4, 111.0, 111.1, 113.7, 117.5, 119.5, 120.5, 121.5, 122.3, 123.6, 126.0, 126.0, 126.1, 127.0, 128.1, 134.1, 134.9, 142.0, 147.2, 152.6, 159.0, 170.5; ESI-HRMS for C₂₄H₂₀N₃O₃ (M+H)⁺ calcd. 398.1505, found 398.1509.

2-(naphthalene-1-yloxy)-N-(2-(pyrimidin-2-yl)benzo[d]oxazol-5-yl)propionamide (67)

63%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.83 (d, J = 6.8 Hz, 3H), 5.04 (q, J = 6.8 Hz, 1H), 6.93 (d, J = 7.6 Hz, 1H), 7.39 (t, J = 8.4 Hz, 1H), 7.61 – 7.53 (m, 4H), 7.88 – 7.86 (m, 1H), 8.09 (s, 1H), 8.34 (d, J = 7.2 Hz, 1H), 8.44 (s, 1H), 8.99 (d, J = 4.8 Hz, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 19.1, 76.4, 107.5, 111.7, 111.7, 113.1, 120.5, 121.5, 122.2, 122.4, 125.9, 126.0, 126.1, 127.0, 128.1, 134.8, 134.9, 142.2, 148.4, 152.7, 155.2, 158.2, 170.7; ESI-HRMS for C₂₄H₁₉N₄O₃ (M+H)⁺ calcd. 411.1457, found 411.1460.

(S)-N-(7-chloro-2-(pyridin-4-yl)benzo[d]oxazol-5-yl)-2-(2,3-dichlorophenoxy)propionamide (68)

68%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.73 (d, J = 6.8 Hz, 3H), 4.87 (q, J = 6.8 Hz, 1H), 6.90 (dd, J₁ = 6.4 Hz, J₂ = 3.2 Hz, 1H), 7.18 – 7.23 (m, 2H), 7.75 (d, J = 2 Hz, 1H), 7.97 (d, J = 1.6 Hz, 1H), 8.08 (d, J = 5.2 Hz, 2H), 8.82 (d, J = 8 Hz, 3H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.5, 77.0, 110.4, 113.3, 116.5, 119.2, 121.3, 124.5, 128.1, 133.7, 134.6, 135.3, 143.2, 144.8, 150.9, 153.6, 162.1, 162.6, 169.3; ESI-HRMS for C₂₁H₁₅N₃O₃Cl₃ (M+H)⁺ calcd. 462.0179 found 462.0182, Chiral purity (% ee >99, t_R= 4.52).

2-methyl-3-phenyl-N-(2-(pyridine-4-yl)benzo[d]oxazol-5-yl)propionamide (69)

75%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.29 (d, J = 6 Hz, 3H), 2.63 (q, J = 6.4 Hz, 1H), 2.77 (dd, J₁ = 16 Hz, J₂ = 5.6 Hz, 1H), 3.02 (dd, J₁ = 16 Hz, J₂ = 9.2 Hz, 1H), 7.17 – 7.25 (m, 5H), 7.34 – 7.44 (m, 3H), 7.75 (s, 1H), 7.99 (d, J = 4 Hz, 2H), 8.75 (s, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.0, 40.9, 44.9, 110.9, 112.5, 120.0, 121.2, 126.7, 128.8, 129.1, 134.4, 135.3, 139.8, 142.1, 147.8, 150.8, 174.3; ESI-HRMS for C₂₂H₂₀N₃O₂ (M+H)⁺ calcd. 358.1556 found 358.1556.

2-phenyl-N-(2-(pyridine-4-yl) benzo[d]oxazol-5-yl)propionamide (70)

77%; ¹H NMR (CDCl₃, 400 MHz) δ 1.61 (d, J = 7.2 Hz, 3H), 3.74 (q, J = 7.2 Hz, 1H), 7.17 (s, 1H), 7.23 (s, 1H), 7.29 – 7.32 (m, 1H), 7.37 – 7.41 (m, 4H), 7.47 (d, J = 8.8 Hz, 1H), 7.91 (d, J = 1.6 Hz, 1H), 8.01 (dd, J₁ = 1.6 Hz, J₂ = 1.2 Hz, 2H), 8.77 (d, J = 5.6 Hz, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.8, 48.3, 110.9, 112.1, 119.4, 121.1, 127.92, 127.94, 129.4, 134.3, 135.4, 140.9, 142.3, 147.8, 150.9, 161.6, 172.5; ESI-HRMS for C₂₁H₁₈N₃O₂ (M+H)⁺ calcd. 344.1399, found 344.1400.

(S)-2-phenyl-N-(2-(pyridine-4-yl)benzo[d]oxazol-5-yl)propionamide (71)

77%; ¹H NMR (CDCl₃, 400 MHz) δ 1.52 (d, J = 7.2 Hz, 3H), 3.67 (q, J = 7.2 Hz, 1H), 7.15 (t, J = 8.4Hz, 1H), 7.21 – 7.35 (m, 5H), 7.69 (s, 1H), 7.83 (s, 1H), 7.91 (d, J = 5.6 Hz, 2H), 8.68 (d, J = 5.2 Hz, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.9, 31.1, 42.7, 48.0, 110.8, 112.2, 119.7, 120.5, 121.1, 127.7, 127.8, 128.1, 129.3, 134.3, 135.6, 141.1, 142.1, 147.7, 148.4, 149.1, 150.8, 161.4, 172.9; ESI-HRMS for C₂₁H₁₈N₃O₂ (M+H)⁺ calcd. 344.1399, found 344.1397, Chiral purity (% ee >99, tR = 8.07 min).

(R)-2-phenyl-N-(2-(pyridine-4-yl)benzo[d]oxazol-5-yl)propionamide (72)

78%; white solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.59 (d, J = 7.2 Hz, 3H), 3.76 (q, J = 6.8 Hz, 1H), 7.21 – 7.29 (m, 1H), 7.32 – 7.41 (m, 6H), 7.90 (s, 1H), 7.97 (d, J = 4.8 Hz, 3H), 8.74 (s, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.9, 48.0, 110.8, 112.3, 119.8, 121.2, 127.7, 127.8, 129.2, 134.3, 135.6, 141.1, 142.1, 147.7, 150.7, 161.4, 172.9; ESI-HRMS for C₂₁H₁₈N₃O₂ (M+H)⁺ calcd. 344.1399, found 344.1393, Chiral purity (% ee 99, t_R=11.29 min).

2-(2,3-dichlorophenylamino)-N-(2-(pyridine-4-yl)benzo[d]oxazol-5-yl)propionamide (73)

71%; ¹H NMR (CDCl₃, 400 MHz) δ 1.72 (d, J = 6.8 Hz, 3H), 3.95 (dq, J₁ = 7.2 Hz, J₂ = 3.2 Hz, 1H), 4.79 (d, J = 2.8 Hz, 1H), 6.56 (d, J = 8.4 Hz, 1H), 6.96 (d, J = 8 Hz, 1H), 7.09 (t, J = 8 Hz, 1H), 7.46 (dd, J₁ = 9.2 Hz, J₂ = 2 Hz, 1H), 7.54 (d, J = 8.4 Hz, 1H), 8.06 (d, J = 4 Hz, 2H), 8.09 (d, J = 1.6 Hz, 1H), 8.56 (s, 1H), 8.8 (bs, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 20.0, 56.4, 110.8, 111.1, 112.4, 118.6, 119.6, 120.9, 121.2, 128.4, 133.5, 134.3, 134.7, 142.4, 144.1, 148.0, 150.9, 161.7, 171.7; ESI-HRMS for C₂₁H₁₇N₄O₂Cl₂ (M+H)⁺ calcd. 427.0729 found 427.0724.

Preparation of (S)-N-methyl-2-(naphthalen-1-yloxy)-N-(2-(thiazol-5-yl)benzo[d]oxazol-5-yl)propanamide (9, R₁ = 5-thiazole, R₂ = 1-naphthyl, R₃= (S)-Me, Y = O, X = H)

A solution of 63 (30 mg, 0.072 mmol) in 2 mL anhydrous THF was added drop-wise to a suspension of sodium hydride (60% in mineral oil, 1.89 mg, 0.079 mmol) in anhydrous THF (3 mL) under a nitrogen atmosphere at 0 °C. The resulting mixture was stirred for 30 min at room temperature. The mixture was cooled to 0 °C and then MeI (11.2 mg, 0.079 mmol) was added. The reaction mixture was stirred for 1 h at room temperature. The reaction mixture was quenched with a few drops of water and extracted with DCM. The organic layers were washed with brine, dried over anhydrous MgSO₄, filtered, and concentrated. The residue was purified by silica gel column chromatography using ethyl acetate/n-hexane (40:60) to yield 9 (15 mg, 48%) as a white solid. ¹H NMR (CDCl₃, 400 MHz) δ 1.62 (d, J = 6.4 Hz, 3H), 3.34 (s, 3H), 4.92 (q, J = 6.4 Hz, 1H), 6.49 (d, J = 7.6 Hz, 1H), 7.03 (d, J = 8.4 Hz, 1H), 7.19 – 7.39 (m, 6H), 7.69 (d, J = 8 Hz, 1H), 7.90 (d, J = 8.4 Hz, 1H), 8.61 (s, 1H), 8.61 (s, 1H), 8.98 (s, 1H); ¹³C NMR (CDCl3, 100 MHz) δ 18.7, 39.0, 72.6, 105.9, 111.4, 119.0, 121.3, 122.3, 125.2, 125.29, 125.3, 125.5, 126.0, 126.5, 127.4, 134.7, 140.0, 142.6, 146.0, 149.7, 153.0, 156.9, 158.4, 171.3; ESI-HRMS for C24H20N3O3S (M+H)+ calcd. 430.1225, found 430.1222, Chiral purity (% ee >99, tR=12.2 min.

Preparation of (S)-2-((2,3-dichlorophenyl)amino)propionoic acid (11)

A mixture of 10 (163 mg, 1.82 mmol), 2,3-dichloro iodobenzene (500 mg, 1.83 mmol), Cs2CO3 (1.19 gr, 3.65 mmol), and CuI (69.7 mg, 0.36 mmol) in DMF (3 mL) under a nitrogen atmosphere was heated at 90 °C for 48 h. The mixture was allowed to cool to room temperature and then diluted with water and the pH was adjusted to 3 to 5 by the addition of concentrated HCl. The mixture was extracted with DCM. The organic layers were washed with brine, dried over anhydrous MgSO4, filtered, and concentrated. The residue was purified by silica gel flash column chromatography using ethyl acetate/n-hexane (50:50) to afforded 11 (240 mg, 57%) as a brown solid. ¹H NMR (CDCl₃, 400 MHz) δ 1.60 (d, J = 7.2 Hz, 3H), 4.17 (q, J = 7.2 Hz, 1H), 6.48 (d, J = 8 Hz, 1H), 6.85 (d, J = 8 Hz, 1H), 7.05 (t, J = 8.4 Hz, 1H).

Synthesis of (S)-2-(2,3-dichlorophenylamino)-N-(2-(pyridine-4-yl)benzo[d]oxazol-5-yl) propionamide (15a)

The general procedure for 8 was followed using 11 and 4 to produce 15a; 70%; brown solid; ¹H NMR (CDCl₃, 400 MHz) δ 1.72 (d, J = 6.8 Hz, 3H), 3.95 (dq, J₁ = 7.2 Hz, J₂ = 3.2 Hz, 1H), 4.79 (d, J = 2.8 Hz, 1H), 6.56 (d, J = 8.0 Hz, 1H), 6.96 (d, J = 8 Hz, 1H), 7.09 (t, J = 8 Hz, 1H), 7.47 (d, J = 8.4 Hz, 1H), 7.54 (d, J = 8.8 Hz, 1H), 8.08 (bd, 3H), 8.56 (s, 1H), 8.8 (bs, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 19.9, 56.35, 110.7, 111.0, 112.5, 118.5, 119.7, 120.7, 121.2, 128.3, 133.5, 134.3, 134.8, 142.3, 144.1, 148.0, 150.8, 161.6, 171.9; ESI-HRMS for C₂₁H₁₇N₄O₂Cl₂ (M+H)⁺ calcd. 427.0729 found 427.0731, Chiral purity (% ee 90.5, t_R = 20.4 min).

Preparation of (S)-methyl 2-(naphthalen-1-ylamino)propanoate (13)

Into a dry round bottom flask equipped with a stir bar was added L-alanine methyl ester hydrochloride (500 mg, 3.58 mmol), 1-naphthalene boronic acid (1000 mg, 5.81 mmol), Cu(OAc)₂ (715 mg, 3.93 mmol), and 4 Å molecular sieves (1.34 g). The flask was sealed with a septum, evacuated and back filled with oxygen. Triethylamine (0.92 mL) and dry DCM (30 mL) were added. The reaction mixture was stirred at room temperature for 48 h. The reaction mixture was quenched with 13 mL 2M NH₃ in methanol. The volatiles were removed in vacuo and the resulting crude oil was purified by silica gel flash chromatography using ethyl acetate/n-hexane (10:90) to give 13 (280 mg, 34%) as a brown viscous oil. ¹H NMR (CDCl₃, 400 MHz) δ 1.27 (t, J = 6.8 Hz, 3H), 1.61 (d, J = 6.8 H, 3H), 4.23 (q, J = 6.8 Hz, 2H), 4.31 (q, J = 6.4 Hz, 1H), 4.98 (bs, NH), 6.55 (d, J = 7.2 Hz, 1H), 7.27–7.35 (m, 2H), 7.45–7.49 (m, 2H), 7.78–7.81 (m, 1H), 7.90–7.93 (m, 1H).

Preparation of N-1-naphthalenyl L-alanine (14)

Ester 13 (40 mg, 0.174 mmol) was dissolved in methanol (1mL) and 1M NaOH in aqueous solution (0.18 mmol, 1.1 eq) was added drop-wise. The reaction mixture was stirred at room temperature for 12 h and then concentrated. The residue was dissolved in DCM and then extracted with 10% aqueous Na₂CO₃ solution. The aqueous layer was acidified with 1M HCl. The precipitate was collected and washed with DCM. The solid was purified by silica gel flash column chromatography using ethyl acetate/n-hexane (40:70) to give 14 (25 mg, 67%) as a brown solid. ¹H NMR (CDCl₃, 400 MHz) δ 1.60 (d, J = 6.8 Hz, 3H), 4.24 (q, J = 6.8 Hz, 1H), 6.49 (d, J1 = 6 Hz, J2 = 1.6 Hz, 1H), 7.18–7.25 (m, 2H), 7.39–7.41 (m, 2H), 7.72–7.81 (m, 2H).

Synthesis of (S)-2-(naphthalen-1-ylamino)-N-[2-(pyridin-4-yl)benzo[d]oxazol-5-yl]propionamide (15b)

The general procedure for 8 was followed using 14 and 4 to produce 15b (60%) as a brown solid. ¹H NMR (CDCl₃, 400 MHz) δ 1.79 (d, J = 6.8 Hz, 3H), 4.13 (q, J = 6.8 Hz, 1H), 4.69 (bs, 1H), 6.66 (d, J = 7.2 Hz, 1H), 7.56- 7.32 (m, 6H), 7.87 (d, J = 7.2 Hz, 1H), 7.95 (d, J = 8 Hz, 1H), 8.08 (bs, 3H), 8.79 (bs, 3H); ¹³C NMR (CDCl₃, 100 MHz) δ 20.2, 56.4, 106.9, 110.9, 112.4, 119.6, 12.7, 120.3, 121.1, 123.6, 125.7, 126.3, 126.6, 129.2, 134.3, 134.4, 135.0, 141.4, 142.3, 147.9, 150.9, 161.6, 172.4; ESI-HRMS for C₂₅H₂₀N₄O₂ (M+H)₊ calcd. 409.1586 found 409.1668, Chiral purity (% ee 97, t_R = 36.4 min).

Preparation of 2-(2,3-dichlorophenyl)acetyl chloride (17)

2,3-Dichlorophenylacetic acid (16, 500 mg, 2.43 mmol) was dissolved in thionyl chloride (4 mL) under a nitrogen atmosphere at 0 °C. The reaction mixture was heated at 90 °C for 2 h. The excess thionyl chloride was removed in vacuo to afford 17 as a colorless liquid. This material was used without further purification.

Preparation of (R)-4-benzyl-3-(2-(2,3-dichlorophenyl)acetyl)oxazolidin-2-one (19)

(R)-4-benzyloxazolidin-2-one (18, 212.6 mg, 1.19 mmol) was dissolved in anhydrous THF (8 mL) under a nitrogen atmosphere. The reaction mixture was cooled to −78 °C and then a 2.5M solution of n-butyl lithium in hexanes (0.9 mL, 1.2 mmol) was added drop-wise. After 1 h, 17 (492 mg, 2.4 mmol) was added. The reaction mixture was stirred for 15 min −78 °C. Then the reaction mixture was allowed to warm to 0 °C and stirred for 30 min. The reaction mixture was then quenched with saturated aqueous NH₄Cl solution. The solvent was removed in vacuo, and then the mixture was extracted with DCM. The organic layer was washed with brine, dried over anhydrous MgSO_4, filtered, and concentrated. The residue was purified by silica gel column chromatography using EtOAc/n-hexane (20:80) to give 19 (350 mg, 80%) as a brown semi-solid. ¹H NMR (CDCl₃, 400 MHz) δ 2.77 (t, J = 12 Hz, 1H), 3.29 (d, J = 12.8 Hz, 1H), 4.23 (m,2H), 4.35 (d, J = 18.4 Hz, 1H), 4.47 (d, J = 18.4 Hz, 1H), 4.67 (m,1H), 7.16–7.30 (m, 7H), 7.40 (dd, J₁ = 4 Hz, J₂ = 2.4 Hz, 1H).

Preparation of (4R)-4-benzyl-3-(2-(2,3-dichlorophenyl)propanoyl)oxazolidin-2-one (20)

To a solution of 19 (250 mg, 0.686 mmol) in anhydrous THF (10 mL) was added sodium bis(trimethylsilyl)amide (0.61 mL, 0.617 mmol) at −78 °C under a nitrogen atmosphere. After 1 h, methyl iodide (0.192 mL, 3 mmol) was slowly added. The reaction mixture was stirred for 2 h at −78 °C and then allowed to warm to room temperature over 5 h. Reaction mixture was quenched with saturated aqueous NH₄Cl. The mixture was diluted with DCM. The organic layer was washed sequentially with water, saturated sodium sulfite and brine. The organic phase was dried over anhydrous MgSO₄, filtered and concentrated. The residue was purified by silica gel chromatography eluting with a linear gradient ranging from 5 to 20% ethyl acetate in n-hexane to provide 20 (200 mg, 77%) as a white foam. ¹H NMR (CDCl₃, 400 MHz) δ 1.56 (d, J = 6.8 Hz, 3H), 2.79 (t, J = 12.0 Hz, 1H), 3.28 (d, J = 13.6 Hz, 1H), 4.09–4.16 (m, 2H), 4.66 (bs, 1H), 5.37 (q, J = 6.8 Hz,1 H), 7.19–7.37 (m, 8H).

Preparation of (R)-2-(2,3-dichlorophenyl)propionic acid (21)

To a solution of 20 (160 mg, 0.42 mmol) in THF (5 mL) and water at 0 °C, was added drop-wise a solution of lithium peroxide [prepared by adding 30% hydrogen peroxide (2.9mL, 2.10 mmol) to lithium hydroxide (17.6 mg, 0.41 mmol) in water (0.679 mL)]. The reaction mixture was stirred for 0 °C for 1 h, and then quenched with saturated aqueous Na₂SO₃ (1.28 mL). The solvent was removed in vacuo. The residue was diluted with water and then extracted with DCM (2 times). The aqueous layer was acidified with concentrated HCl and then extracted with EtOAc (2 times). The EtOAc extracts were combined, washed with brine, dried over anhydrous MgSO₄, and concentrated. The residue was purified by silica gel column chromatography using EtOAc/n-hexane (40:60) to afforded 21 (80 mg, 87%) as a white solid. ¹H NMR (CDCl₃, 400 MHz) δ 1.51 (d, J = 7.2 Hz, 3H), 4.27 (q, J = 6.8 Hz, 1H), 7.16–7.24 (m, 2H), 7.37(d, J = 7.6 Hz, 1H).

Synthesis of (R)-2-(2,3-dichlorophenyl-N-(2-(pyridine-4-yl)benzo[d]oxazol-5-yl)propionamide (22)

The general procedure for 8 using 21 and 4 yielded 22 as a white solid (70%); ¹H NMR (CDCl₃, 400 MHz) δ 1.61 (d, J = 7.2 Hz, 3H), 4.31 (q, J = 6.8 Hz, 1H), 7.26 (t, J = 8 Hz, 1H), 7.42 – 7.54 (m, 4H), 7.98 (s, 1H), 8.05 (d, J = 4.4 Hz, 2H), 8.80 (d, J = 4 Hz, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 17.8, 45.0, 111.0, 112.4, 119.7, 121.2, 126.8, 128.2, 129.7, 132.0, 133.7, 134.3, 135.4, 140.9, 142.3, 147.9, 150.8, 161.6, 171.3. ESI-HRMS for C₂₁H₁₅Cl₂N₃O₂ (M+H)⁺ calcd. 412.0620, found 412.0611, Chiral purity (% ee 83, t_R = 1.58 min).

Synthesis of 5-nitro-2-(pyridine-4-yl)-1H-benzo[d]imidazole (24)

4-Nitro-1,2-phenylenediamine (23, 500 mg, 3.26 mmol) and 4-pyridine carboxaldehyde (419 mg, 3.91 mmol) were dissolved in DMF (10 mL). Disodium metabisulfite (742 mg, 3.91 mmol) was added. The reaction mixture was heated at 120 °C for 24 h under a nitrogen atmosphere. After allowing the mixture to cool to room temperature, volatiles were removed under reduced pressure. The reaction mixture was then diluted with water and extracted from DCM. The organic layer was dried on anhydrous MgSO₄, filtered and concentrated. The residue was purified by silica gel column chromatography using MeOH/CHCl₃ (5:95) to give 24 (480 mg, 61%) as a red solid. ¹H NMR (CDCl₃, 400 MHz) 8.7 (d, J = 9.2 Hz, 2H), 8.1 (d, J = 2.4 Hz, 1H), 8.03 (d, J = 8.2 Hz, 2H), 7.92 (dd, J₁ = 8.0 Hz, J₂ = 2.4 Hz, 1H), 7.82 (s, 1H).

Synthesis of 2-(pyridin-4-yl)-1H-benzo[d]imidazole-5-amine (25)

Substrate 24 was reduced by hydrogenation using the general procedure described for 4 to yield 25 (64%). ¹H NMR (CDCl₃, 400 MHz) 8.50 (d, J = 8.2 Hz, 2H), 7.44 (d, J = 8.2 Hz, 2H), 6.70 (dd, J₁ = 8 Hz, J₂ = 2Hz, 1H), 6.20–6.17 (dd, J₁= 8 Hz, J₂ = 2 Hz, 1H), 6.10 (s, 1H), 4.30 (s, 2H).

Preparation of 2-phenoxy-N-[2-pyridin-4-yl)-1H-benzo[d]imidazole-5-yl]propionamide (26)

To a solution of 25 (50 mg, 0.237 mmol) in anhydrous THF (6 mL) was added 2-phenoxypropionyl chloride (52.5 mg, 0.284 mmol), TEA (36.2 mg, 0.355 mmol), 4-DMAP (2.89 mg, 0.023 mmol) at 0 °C. The reaction mixture was stirred for 30 min at room temperature and quenched with sodium bicarbonate solution. The reaction mixture was diluted with DCM and washed with sodium bicarbonate solution followed by brine solution. The organic layer was dried over anhydrous MgSO₄, filtered and concentrated. The residue was purified by silica gel column chromatography using MeOH/CHCl₃ (10:90) to furnish 26 (60 mg, 70%) as a brown solid. ¹H NMR (CDCl₃, 400 MHz) δ 1.68 (d, J = 6.8 Hz, 3H), 4.85 (q, J = 6.4 Hz, 1H), 6.95 – 7.05 (m, 4H), 7.26 – 7.32 (m, 2H), 7.52 (d, J = 8.4 Hz, 1H), 7.86 (d, J = 5.2 Hz, 2H), 8.08 (s, 1H), 8.57 (d, J = 5.2 Hz, 2H), 8.65 (s, 1H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.9, 75.5, 115.9, 117.72, 117.74, 120.8, 122.8, 130.1, 132.8, 137.6, 150.0, 150.3, 156.7, 171.5; ESI-HRMS for C₂₁H₁₆N₃O₃Cl₂ (M+H)₊ calcd. 428.0569 found 428.0578.

Synthesis of (but-3-yn-2-yloxy)benzene (28)

Phenol (27, 422.9 mg, 4.49 mmol) and but-3-yn-2-ol (300 mg, 4.28 mmol) were dissolved in anhydrous THF (10 mL) under a nitrogen atmosphere at 0 °C. Then Ph₃P (1.12 g, 4.26 mmol) was added portion-wise. The reaction mixture was stirred for 10 min and then DEAD (894.6 mg, 5.14 mmol) was slowly added. The resulting solution was heated at 70 °C for 20 h. The reaction mixture was allowed to cool to room temperature and then water was added. The mixture was extracted with DCM. The organic layer was dried over anhydrous MgSO₄, filtered, and concentrated. The residue was purified by silica gel flash chromatography using ethyl acetate/n-hexane (5:95) providing 28 (450 mg, 69%) as a solid. ¹H NMR (CDCl₃, 400 MHz) δ 1.67 (d, J = 6.8 Hz, 3H), 2.47 (d, J = 2 Hz, 1H), 4.88 (q, J = 2 Hz, 1H), 6.97 – 7.03 (m, 3H), 7.28 – 7.32 (m, 2H).

Preparation of 5-azido-2-(thiazol-5-yl)benzo[d]oxazole (29)

A solution of 2-(thiazol-5-yl)benzo[d]oxazol-5-amine (4, R₁ = 5-thiazole, X = H, 50 mg, 0.23 mmol) dissolved in 2 mL concentrated HCl:H₂O (1:1) was cooled at −5 °C. Then a solution of sodium nitrite (31.7 mg, 0.459 mmol) dissolved in water (15 mL) was slowly added. The reaction mixture was stirred for 60 min and then neutralized with sodium acetate (37.7 mg, 0.459 mmol). Next, a solution of NaN₃ (29.9 mg, 0.49 mmol) in water (0.5 mL) was slowly added over a 30 min period maintaining a temperature between 0 – 5 °C. After additional stirring for 30 min, the solution was allowed to warm to room temperature and then it was extracted with ethyl acetate. The organic layer was dried over anhydrous MgSO₄, filtered and concentrated to yield 29 (50 mg, 89%) as a solid, which was used without further purification.

Preparation of 5-(4-(1-phenoxyethyl)-1H-1,2,3-triazol-1-yl)-2-(thiazol-5-yl)benzo[d]oxazole (30)

A mixture of 29 (37 mg, 0.15 mmol) and 28 (20 mg, 0.13 mmol) were dissolved in anhydrous acetonitrile (3 mL) under a nitrogen atmosphere. Then DIPEA (53 mg, 0.40 mmol) was added and the mixture stirred at room temperature for 10 min. Next, CuI (51.7 mg, 0.27 mmol) was added portionwise and then the reaction mixture was stirred for 40 min. The mixture was quenched with aqueous saturated NH₄Cl, diluted with water and extracted with DCM. The organic layer was dried over anhydrous MgSO_4, filtered and concentrated. The residue was purified by silica gel column chromatography using ethyl acetate/n-hexane (50:50) to furnish 30 (43 mg, 84%) as a solid. ¹H NMR (CDCl₃, 400 MHz) δ 1.77 (d, J = 6.4 Hz, 3H), 5.69 (q, J = 6.4 Hz, 1H), 6.90 – 6.97 (m, 3H), 7.22 – 7.24 (m, 2H), 7.66 (d, J = 8.8 Hz, 1H), 7.74 (dd, J₁ = 8 Hz, J₂ = 2 Hz, 1H), 7.90 (s, 1H), 7.99 (d, J = 2 Hz, 1H), 8.66 (s, 1H), 8.98 (s, 1H); ¹³C NMR (Pyridine-d₅, 100 MHz) δ 22.1, 69.5, 112.3, 112.5, 116.6, 119.1, 121.7, 121.8, 125.8, 130.3, 135.3, 143.2, 146.9, 150.5, 151.1, 158.5, 159.0, 159.3; ESI-HRMS for C₂₀H₁₆N₅O₂S (M+H)⁺ calcd. 390.1025, found 390.1031.

Preparation of N-(3-nitrophenyl)isonicotinamide (32)

The general procedure for 8 was followed using 3-nitroaniline (31) and isonicotinic acid to afforded 32 (68%) as a yellow solid. ¹H NMR (DMSO-d₆, 400 MHz) δ 7.67 – 7.71 (m, 1H), 7.89 – 7.91 (m, 2H), 8.01 (dd, J₁ = 8.4 Hz, J₂ = 1.6Hz, 1H), 8.19 (dd, J₁ = 8 Hz, J₂= 1.6 Hz, 1H), 8.79–8.83 (m, 3H), 10.9 (NH, s, 1H).

Preparation of N-(3-aminophenyl)isonicotinamide (33)

N-(3-nitrophenyl)isonicotinamide (32, 0.823 mmol) in 4 mL ethanol was heated to 70 °C. Then SnCl₂•H₂O (4.2 mmol) was added in portions and refluxed until starting material disappeared (~ 4 h). The reaction mixture was cooled and quenched with saturated sodium bicarbonate solution, and extracted with ethyl acetate (3 × 10 mL). The organic layer was washed with saturated sodium bicarbonate, brine, dried over anhydrous sodium sulfate, filtered and concentrated in vacuo to give a residue, which was used without further purification.

Preparation of (S)-N-(3-(2-(2,3-dichlorophenoxy)propanamido)phenyl)isonicotinamide (34)

The general procedure for 8 was followed using 33 and 7 (R₂ = 2,3-di-ClPh, Y = O, R₃ = (S)-Me) to afford 34 (63%) as a white solid. ¹H NMR (CDCl₃, 400 MHz) δ 1.67 (d, J = 6.8 Hz, 3H), 4.79 (q, J = 6.8 Hz, 1H), 6.84 – 6.88 (m, 1H), 7.15 – 7.23 (m, 3H), 7.31 (t, J = 8 Hz, 1H), 7.59 (d, J = 8 Hz, 1H), 7.68 (d, J = 4.4 Hz, 2H), 8.02 (s, 1H), 8.24 (s, 1H), 8.68 (s, 1H), 8.75 (s, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.5, 77.0, 111.9, 113.3, 116.5, 117.0, 121.2, 123.0, 124.4, 128.0, 130.0, 134.5, 137.8, 138.3, 142.1, 150.8, 153.7, 164.0, 169.5; ESI-HRMS for C₂₁H₁₈N₃O₃Cl₂ (M+H)⁺ calcd. 430.0725 found 430.0725, Chiral purity (% ee >99, t_R=10.54).

Synthesis of N-(1-phenylethyl)2-pyridin-4-yl)benzo[d]oxazole-5-carboxamide (36)

The general procedure for 8 was followed using 35 and α-methyl benzylamine to give 36 (71%) as a white solid. ¹H NMR (CDCl₃, 400 MHz) δ 1.57 (d, J = 6.8 Hz, 3H), 5.29 (q, J = 6.8 Hz, 1H), 6.41 (d, J = 7.2 Hz, 1H), 7.19–7.25 (m, 1H), 7.29–7.36 (m, 4H), 7.58 (d, J = 8.4 Hz, 1H), 7.85 (d, J = 8.4 Hz, 1H), 8.00 (d, J = 4.8 Hz, 2H), 8.13 (s, 1H), 8.77 (bs, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 21.9, 49.7, 111.2, 119.6, 121.3, 126.0, 126.4, 127.8, 129.0, 132.4, 134.0, 142.0, 143.1, 151.0, 152.8, 162.1, 166.1; ESI-HRMS for C₂₁H₁₈N₃O₂ (M+H)⁺ calcd. 344.1399 found 344.1403.

Preparation of 6-nitro-2-(pyridin-4-yl)benzo[d]oxazole (38a)

The general procedure for 3 was followed using 2-amino-5-nitrophenol (37) and 4-pyridyl carboxaldehyde to give 38a (75%) as a yellow solid. ¹H NMR (DMSO-d₆, 400 MHz) 6.99–7.05 (m, 1H), 8.13 (d, J= 5.2 Hz, 2H), 8.36 (d, J= 8.4 Hz, 1H), 8.89 (d, J = 7.4 Hz, 2H), 8.81 (s, 1H).

Preparation of 2-(pyridin-4-yl)benzo[d]oxazol-6-amine (39a)

The general procedure for 4 was followed using 38a to obtained 39a (85%) as a yellow solid. ¹H NMR (DMSO-d₆, 400 MHz) 5.61 (s, 2H), 6.66 (dd, J₁ = 8 Hz, J₂ = 2 Hz, 1H), 6.79 (d, J = 1.6 Hz, 1H), 7.44 (d, J = 8.8 Hz, 1H), 7.92 (d, J = 6 Hz, 2H), 8.71 (d, J = 6 Hz, 2H).

(S)-2-(2,3-dichlorophenoxy)-N-(2-(pyridin-4-yl)benzo[d]oxazol-6-yl)propionamide (40a)

The general procedure for 8 was followed using 7 (R₂ = 2,3-di-ClPh, Y=O, R₃=(S)-Me) and 39a to afford 40a (76%) as a white solid. ¹H NMR (CDCl₃, 400 MHz) δ 1.71 (d, J = 6.8 Hz, 3H), 4.85 (q, J = 6.6 Hz, 1H), 6.88 (dd, J₁ = 7.2 Hz, J₂ = 2.8 Hz, 1H), 7.12 – 7.18 (m, 2H), 7.24 (dd, J₁ = 8.8 Hz, J₂ = 2 Hz, 2H), 7.69 (d, J = 8.4 Hz, 1H), 7.99 (d, J = 5.6 Hz, 2H), 8.33 (s, 1H), 8.75 (d, J = 5.2 Hz, 2H), 8.91 (s, 1H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.3, 76.7, 102.6, 113.0, 117.4, 120.5, 120.7, 122.6, 124.1, 127.8, 134.0, 134.2, 135.6, 138.3, 150.6, 151.1, 153.4, 160.8, 169.0; ESI-HRMS for C₂₁H₁₆N₃O₃Cl₂ (M+H)⁺ calcd. 428.0569, found 428.0567, Chiral purity (% ee >99, t_R=21.0).

(S)-2-(2,3-dichlorophenoxy)-N-(2-(thiazol-5-yl)benzo[d]oxazol-6-yl)propionamide (40b)

A similar sequence of reactions that was used to prepare 40a was also used to generate 40b as a white solid. ¹H NMR (CDCl₃, 400 MHz) δ 1.73 (d, J = 6.8 Hz, 3H), 4.88 (q, J = 6.4 Hz, 1H), 6.89 – 6.92 (m, 1H), 7.17 – 7.19 (m, 1H), 7.23- 7.26 (m, 2H), 7.66 (d, J = 8.8 Hz, 1H), 8.29 (s, 1H), 8.61 (s, 1H), 8.85 (s, 1H), 8.94(s, 1H); ¹³C NMR (CDCl₃, 100 MHz) δ 18.5, 76.9, 102.8, 113.2, 117.5, 120.1, 122.9, 124.3, 125.7, 128.1, 134.5, 135.4, 138.6, 142.4, 151.0, 153.6, 153.68, 156.2, 157.3, 169.2; ESI-HRMS for C₁₉H₁₄N₃O₃SCl₂ (M+H)⁺ calcd. 434.0133, found 434.0139, Chiral purity (% ee >99, t_R= 22.5).

Gene cloning, protein expression, purification and crystallization

The coding sequence of CpIMPDH enzyme was amplified by PCR from CpIMPDH-90-134-pET28a plasmid^10b using primers compatible with the ligation independent cloning vector pMCSG7.²¹ The gene was cloned into pMCSG7 using a modified ligation-independent cloning protocol.²² The recombinant construct produced fusion proteins with an N-terminal His₆-tag and a TEV protease recognition site (ENLYFQ↓S). The fusion protein was expressed in an E. coli strain BL21(DE3) harboring pMAGIC plasmid encoding one rare E. coli Arg tRNAs (covering codons AGG/AGA) in the presence of 100 μg/mL ampicillin and 30 μg/mL kanamycin. The cells were grown in enriched M9 media at 37 °C followed by an overnight induction with 0.5 mM isopropyl-β-D-thiogalactoside (IPTG) at 18 °C. Cells were harvested, resuspended in lysis buffer (50 mM HEPES pH 8.0, 500 mM KCl, 20 mM imidazole, 10 mM 2-mercaptoethanol, 5% glycerol) and stored at –80° C.

CpIMPDH protein was purified according to a standard protocol.²² The protocol included cell lysis by sonication, Ni²⁺-affinity chromatography on an ÄKTAxpress system (GE Healthcare Life Sciences) followed by His₆-tag cleavage using recombinant TEV protease₂₃ and an additional Ni2+-affinity chromatography performed to remove the protease, the uncut protein, and the affinity tag. In the final step, the protein was dialyzed against crystallization buffer containing 20 mM HEPES pH 8.0, 150 mM KCl and 1 mM TCEP, concentrated, flash frozen, and stored in liquid nitrogen. Crystallizations were set up with the help of a Mosquito liquid dispenser (TTP LabTech) using the sitting-drop vapor-diffusion method in 96-well CrystalQuick plates (Greiner Bio-One). For each condition, 0.4 μl protein solution and 0.4 μl crystallization formulation were mixed and equilibrated against a 135 μl reservoir. Five crystallization screens were used. Crystals with IMP and 54 were obtained from a drop solution containing 6 mg/ml protein, 3 mM IMP and 0.5 mM 54. Diffraction quality crystals appeared at 18 °C in 0.1 M succinic acid pH 7.0 and 15% (w/v) PEG 3350. The crystals were mounted on CryoLoops (Hampton Research) and flash-cooled in liquid nitrogen. The cryoprotectant consisted of 20% ethylene glycol.

Data collection and Structure determination

Diffraction data were collected at 100 K at the 19-ID beamline of the Structural Biology Center at the Advanced Photon Source, Argonne National Laboratory.²⁴ The single wavelength data at 0.97923 Å (12.661406 keV) up to 2.1 Å were collected from a single crystal of CpIMPDH complexed with IMP and 54. The crystal was exposed for 5 s per 1.0° rotation of ω with the crystal to detector distance of 300 mm. The data were recorded on a CCD detector Q315 from ADSC scanning 220°. The SBC-Collect program was used for all data collection and visualization. Data collection strategy, integration, and scaling were performed with the HKL3000 program package.²⁵ A summary of the crystallographic data can be found in Table S1.

The structure was determined by molecular replacement using chain A of the structure of CpIMPDH (PDB ID 3FFS) as a search model with HKL3000 using the data to 3.0 Å.²⁵ The initial model contained 4 copies of the search model and there was extra electron density for additional protein residues that were not part of the search model. The presence of IMP and 54 in the active site was apparent from the initial electron density map (F_o). Extensive manual model building with coot²⁶ and the subsequent refinement using phenix.refine²⁷ was performed against the full data set up to 2.1 Å until the structure converged to the R factor (R_work) of 0.162, and R_free of 0.210 with the r.m.s. bond distances of 0.007 and the r.m.s. bond angles of 1.104°. The asymmetric unit contains four protein chains, A, B, C and D comprised of residues 1–92 and 135–392 (chains A and C) and 2–92 and 135–392 (chains B and D). The final model also includes four IMP molecules, four 54 molecules, five ethylene glycol molecules and 486 ordered water molecules. Several residues including several N- and C-terminal residues and those introduced as a cloning artifact (SNA)²⁸ are missing due to disorder. In addition, residues 312–325 of chain A, 309–235 of chain B, 311–325 of chains C and D located on the surface of the tetramer were disordered and not modeled. The stereochemistry of the structure was checked with PROCHECK²⁹ and the Ramachandran plot. Atomic coordinates and experimental structure factors of the structure have been deposited in the PDB under the ID code 4IXH.

Inhibition of recombinant CpIMPDH

Recombinant CpIMPDH, purified from E. coli,¹² was assessed by monitoring the production of NADH by fluorescence at varying inhibitor concentrations (25 pM – 5 YM). IMPDH was incubated with inhibitor for 5 min at room temperature prior to addition of substrates. The following conditions were used: 50 mM Tris-HCl, pH 8.0, 100 mM KCl, 3 mM EDTA, 1 mM dithiothreitol (assay buffer) at 25 °C, 10 nM CpIMPDH, 300 μM NAD and 150 μM IMP. To characterize the non-specific binding of inhibitors, assays were also carried out in the presence of 0.05% BSA (fatty acid free). IC₅₀ values were calculated for each inhibitor according to Equation 1 using the SigmaPlot program (SPSS, Inc.):

$${\upsilon }_{\text{i}}={\upsilon }_{\text{o}}/(1+[\text{I}]/{\text{IC}}_{50})$$ (Eq. 1)

where υ_i is initial velocity in the presence of inhibitor (I) and υ_o is the initial velocity in the absence of inhibitor. When the values of IC₅₀ approach the concentration of enzyme, tight binding treatment is used:

$${\upsilon }_{\text{i}}/{\upsilon }_{\text{o}}=1-(([\text{E}]+[\text{I}]+{\text{IC}}_{50})-{({([\text{E}]+[\text{I}]+{\text{IC}}_{50})}^2-4[\text{E}][\text{I}])}^{0.5})/2[\text{E}]$$ (Eq. 2)

Inhibition at each inhibitor concentration was measured in quadruplicate and averaged; this value was used as υ_i. The IC₅₀ values were determined three times; the average and standard deviations are reported. Mechanism of inhibition was determined by fitting to equations for competitive, noncompetitive/mixed and uncompetitive inhibition as previously described.^16c

Figure 2.

Inhibition of CpIMPDH by 68. Inhibitor concentrations: open squares, 630 nM; closed squares, 380 nM; closed triangles, 130 nM; open triangles, 50 nM; open circles, 25 nM; no inhibitor, closed circles.

Scheme 4.

Synthesis of a benzimidazole derivative.

Reagents and conditions: (a) 4-PyCHO, Na₂S₂O₅, DMF, 120 °C, 24 h, 61% (b) H₂ (1 atm), 10% Pd-C, MeOH, rt, 6 h, 64% (c) PhOCH(CH₃)COCl, TEA, cat. 4-DMAP, THF, 0 °C – rt, 30 min, 70%. A benzimidazole analogue of 1 was prepared using the method outlined in Scheme 4. 6-Nitro-2-(pyridin-4-yl)-1H-benzo[d]imidazole (24) was prepared by condensation of 4-pyridine carboxaldehyde with 4-nitro-1,2-phenylenediamine (23) in the presence of the oxidizing agent sodium metabisulphite.^10d The product was reduced in the presence of hydrogen (1 atm) and 10% Pd-C to give 25, which was then coupled with 2-(phenoxy)propionoyl chloride in the presence of triethylamine and catalytic DMAP to give 26.

Scheme 7.

Synthesis of amide derivative 36.

Reagents and conditions: (a) α-methyl benzyl amine, EDCI•HCl, DMF, 0 °C - rt, 12 h, 71%. An analogue of 22 in which the amide group is inverted was prepared using the method outlined in Scheme 7. Amide 36 was generated by coupling 2-(pyridin-4-yl)benzo[d]oxazole-5-carboxylic acid (35) and α-methylbenzylamine in the presence of EDCI•HCl.

Abbreviations

Cp

Cryptosporidium parvum

CL_int

intrinsic clearance

BSA

bovine serum albumin

DCM

dichloromethane

DEAD

Diethylazodicarboxylate

DIPEA

diisopropylethylamine

DMF

N,N-dimethylformamide

EDCI•HCl

1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride

IMP

inosine 5′-monophosphate

IMPDH

inosine 5′-monophosphate dehydrogenase

hIMPDH

human IMPDH

HTS

high-throughput screening

MPA

mycophenolic acid

MR

molecular replacement

N.D

not determined

r.m.s

root-mean-square

TEA

triethylamine

Toxo

Toxoplasma

XMP

xanthosine 5′-monophosphate